THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 

LOS  ANGELES 


The  RALPH  D.  REED  LIBRARY 

DEPAKTMENT  OP  GBOLOGY 

BNrvERsrrr  of  CALIFORNIA 

LOS  ANGELES,  CALIF. 


American  Mature  Series. 

Group  VI.   The  Philosophy  of  Nature.    Edited  by  V.  L.  Kellogg. 


PLANT    LIFE    AND 
EVOLUTION 


BY 

DOUGLAS   HOUGHTON   CAMPBELL 

Professor  of  Botany  in  Leland  Stanford  Junior  University 


NEW  YORK 

HENRY  HOLT  AND  COMPANY 
1911 


COPYRIGHT,  1911, 

BY 
HENRY  HOLT  AND  COMPANY 

Published  October.  1911 


Geology 
Library 


CONTENTS 

CHAPTER  PACK 

I.    INTRODUCTION 3 

Life  and  Its  Origin — Protoplasm — The  Cell — 
Plants  and  Animals— Regeneration. 

II.    FACTORS  IN  EVOLUTION 19 

Heredity — Environment — Conditions  for  Plant 
Growth — Selection. 

III.  THE  LOWER  PLANTS 47 

Bacteria— Blue-green  Algae — Flagellata — Vol- 
vocales — Green  Algae — Brown  Algae — Red  Al- 
gae— Evolution  of  Sex  in  Algae — Fungi. 

IV.  THE  ORIGIN  OF  LAND  PLANTS       ....      80 

The  Ancestors  of  the  Land  Plants— The  First 
Terrestrial  Plants — Alternation  of  Generations 
— Bryophytes     or     Mosses — Pteridophytes     or 
Ferns — Fossil      Archegoniates — Heterospory — 
Origin  of  Seeds — Fossil  Pteridophytes. 
V.     SEED-PLANTS         .......     120 

Selaginella — The    First    Seed-Plants — Gymno- 
sperms  and  Angiosperms — Cycads — Ginkgoales 
— Coniferales — Gnetales — Resume. 
VI.    THE    ANGIOSPERMS      ......     147 

Fossil  Angiosperms — The  Flower — The  Fruit 
— Monocotyledons  —  Dicotyledons  —  Evolution 
of    the    Flower    in    the    Angiosperms — Cross- 
pollination — Adaptability     of     Angiosperms — 
Summary. 
VII.    ENVIRONMENT  AND  ADAPTATION    ....     186 

Marine  Plants— Terrestrial  Plants — Aquatics — 
Mesophytes — Xerophytes — Halophytes  —  Light 
as  a  Formative  Agent — Fungi — Parasites  and 
Saprophytes — Symbiosis — Plants  and  Animals 
— Reproduction — Cross-pollination, 
iii 


iv  Contents 

CHAPTER  PAGE 

VIII.  THE  PROBLEMS  OF  PLANT  DISTRIBUTION  .  .  236 
Antiquity  of  the  Principal  Plant  Types — Uni- 
formity of  Early  Floras— Origin  of  Angio- 
sperms— Factors  in  Plant  Distribution — Alpine 
Floras — Island  Floras — Agents  in  Plant  Dis- 
tribution— Plant  Distribution  in  the  United 
States. 

IX.    THE  HUMAN  FACTOR  IN  PLANT  EVOLUTION  .         .     277 
Antiquity  of  Cultivated  Plants — Man's  Spread 
over    the    Earth    due    to    Agriculture — Plant 
Breeding. 

X.    THE  ORIGIN  OF  SPECIES 301 

Mutability  of  Plants— Parallel  Development 
of  Reproduction  in  Plants  and  Animals — Vari- 
ation— Orthogenesis — Theories  of  Evolution — 
Changes  Due  to  Environment — The  Mutation 
Theory  of  De  Vries — Heredity — Mendel's  Law 
of  Heredity — Evolution  of  Sex — Significance 
of  Sex — Hybrids — Experimental  Morphology 
— Conclusion. 

INDEX 349 


PLANT   LIFE   AND   EVOLUTION 


PLANT    LIFE    AND    EVOLUTION 

CHAPTER  I 
INTRODUCTION 

SPECULATIONS  concerning  the  origin  of  life 
upon  the  earth,  and  the  nature  of  the  primeval 
organisms  from  which  are  descended  existing  plants 
and  animals  must  always  have  for  the  biologist  an 
irresistible  attraction.  When  we  realize  the  ex- 
traordinary development  of  modern  experimental 
biology,  it  would  be  rash  to  say  that  the  problem 
of  the  origin  of  life  is  insoluble,  but  it  must  be 
admitted  that  its  solution  does  not  seem  to  lie  in 
the  immediate  future. 

Life  and  Its  Origin. — All  living  things  are  built 
up  of  chemical  elements  that  also  occur  in  an  "  in- 
organic "  condition ;  and  the  nature  of  the  combina- 
tions of  these  elements  into  the  substances  that 
make  up  living  matter,  and  the  functions  or  "  life 
processes  "  associated  with  this  living  matter,  are 
the  problems  with  which  the  biologist  has  to  deal. 
Whether  all  of  these  "  vital  "  phenomena  are  re- 
ducible to  terms  of  physics  and  chemistry,  may  per- 
haps be  open  to  question;  but  as  yet  we  have  no 
3 


4  Plant  Life  and  Evolution 

positive  evidence  that  this  is  not  the  case,  and  all 
experimental  work  must  necessarily  be  based  upon 
the  assumption  that  living  matter  is  subject  to  the 
same  laws  that  govern  inorganic  bodies. 

So  far  as  can  be  demonstrated,  all  manifestation 
of  life  is  indissolubly  associated  with  certain  ex- 
tremely complex  "  protei  1  "  substances  which  never 
occur  except  in  living  bodies,  and  which  are  the 
basis  of  the  protoplasm,  or  living  substance,  of  all 
plants  and  animals. 

The  evidence  as  to  the  possibility  of  spontaneous 
generation  at  the  present  time  is  entirely  negative. 
All  of  the  alleged  successful  experiments  in  this 
direction  have  been  shown  to  be  the  result  of  er- 
rors, and  as  yet  no  successful  attempts  have  been 
made  to  produce  living  matter  by  artificial  means. 
So  far  as  we  know  at  the  present  time,  all  living 
things  arise  from  preexisting  ones  by  some  form 
of  reproduction.  It  is,  however,  the  general  belief 
of  biologists,  that  at  some  remote  period  living  mat- 
ter arose  from  inorganic  elements.  The  theory 
that  living  germs  were  brought  to  the  earth  from 
somewhere  in  space  is  not  generally  accepted;  and 
the  criticism  has  been  made  of  this  view  that,  after 
all,  it  does  not  explain  the  origin  of  life,  but  merely 
the  advent  of  life  upon  the  earth. 

Probably  the  most  important  advance  made  in 
recent  years  towards  an  understanding  of  the  na- 
ture of  the  most  primitive  forms  of  life,  has  resulted 
from  the  study  of  certain  bacteria,  which  both 


Introduction  5 

structurally  and  physiologically  must  be  regarded 
as  the  simplest  organisms  of  which  we  have  any 
knowledge.  Especially  are  we  indebted  to  the  re- 
searches of  the  Russian  investigator,  Winogradsky, 
for  information  regarding  these  important  little  or- 
ganisms. These  bacteria  possess  the  remarkable 
power  of  assimilating  the  free  nitrogen  of  the  at- 
mosphere, and  some  of  them,  like  the  higher  green 
plants,  can  decompose  CO2,*  and  they  are  thus  able 
to  live  quite  independently  of  any  organic  food,  a 
condition  of  things  hitherto  supposed  to  be  confined 
to  plants  possessing  the  characteristic  green  pigment 
chlorophyll.  Unlike  the  green  plants  the  assimila- 
tions of  CO2  in  these  bacteria  is  not  dependent  upon 
light;  i.e.,  it  is  not  a  process  of  "photosynthesis." 
We  have  then,  in  these  nitrogen  bacteria,  organ- 
isms of  the  simplest  structure  so  far  as  we  can 
judge,  although  their  excessively  minute  size  may 
account  for  the  failure  to  demonstrate  any  definite 
cell  structure.  In  their  ability  to  assimilate  such 
simple  and  common  substances  as  nitrogen  and 
CO2  they  may  very  well  be  assumed  to  approximate 
the  earliest  forms  of  life  that  appeared  upon  the 
earth.  It  is  reasonable  to  suppose  that  these  primi- 
tive organisms,  like  the  nitrogen  bacteria,  were  able 
to  assimilate  free  nitrogen,  CO2,  and  water,  which 
yield  the  most  important  elements  of  the  proto- 

*  Carbonic  acid  gas.  The  reader  is  urgently  invited  to 
fall  in  with  the  convention  which  now  accepts  this  abbrevia- 
tion quite  generally  in  non-technical  writing. 


6  Plant  Life  and  Evolution 

plasm.  What  were  the  sources  of  energy  for  these 
assimilation  processes,  of  course  we  cannot  tell, 
since  the  requisite  energy  may  have  been  of  various 
kinds  and  derived  from  various  sources, — light, 
heat,  or  chemical  energy,  as  any  of  these  forms  of 
energy  might  very  well  serve  the  same  purpose. 
Thus  the  assimilation  of  CO2  by  the  green  plants 
requires  light  for  its  accomplishment,  but  the  nitro- 
gen bacteria  are  able  to  assimilate  CO2  in  the  ab- 
sence of  light,  probably  through  some  form  of 
chemical  energy. 

When  life  first  appeared  upon  the  earth  the  tem- 
perature was  presumably  very  much  higher  than  at 
present,  and  chemical  activity  would  probably  have 
been  more  active.  It  has  been  supposed  also  that 
CO2  was  more  abundant  in  the  atmosphere  than  at 
the  present  time.  Inasmuch  as  the  assimilation  of 
CO2  by  the  nitrogen  bacteria  is  independent  of  light, 
it  is  not  necessary  to  assume  that  this  assimilation 
in  the  earliest  forms  of  life  was  photosynthetic ; 
but  what  agency  transformed  the  presumably  highly 
unstable  and  complex  inorganic  compounds,  which 
antedated  the  first  living  things,  into  living  proto- 
plasm with  its  power  of  assimilation  and  growth, 
we  do  not  know.  It  may  be  that  further  experi- 
mental work  on  these  simple  living  things,  the 
nitrogen  bacteria,  may  furnish  the  key  to  the  mys- 
tery of  the  origin  of  life. 

The  distinguished  botanist,  Pfeffer,  says :  "  It  re- 
mains uncertain  whether  the  conditions  now  exist- 


Introduction  7 

ing  on  our  globe  are  such  as  to  permit  a  re-creation 
of  life,  or  whether  the  necessary  conditions  were 
presented  once,  and  by  a  special  sequence  of  events 
such  as  we  can  never  hope  to  reproduce.  The  par- 
ticular combination  of  causes,  to  which  the  creation 
of  life  was  possibly  due,  may  have  existed  only 
as  the  earth  cooled  from  its  original  incandescent 
condition  and  perhaps  thereby  caused  certain  essen- 
tial preliminary  stages  in  the  production  of  living 
substance  to  arise." 

Protoplasm. — All  manifestations  of  life  are  di- 
rectly associated  with  living  protoplasm.  When, 
however,  we  attempt  to  define  protoplasm  we  meet 
with  serious  difficulties.  We  know  that  this  viscid, 
more  or  less  granular  matter,  which  constitutes  the 
living  part  of  all  cells,  is  an  enormously  complex 
substance.  Protoplasm  is  in  no  sense  a  definite 
homogeneous  chemical  compound,  but  it  is  a  mix- 
ture of  very  many  presumably  heterogeneous  units, 
which  in  themselves  may  be  of  great  complexity. 
Of  the  real  nature  of  the  units  making  up  the  proto- 
plasmic body  of  the  cell  we  have  very  little  certain 
knowledge.  Various  names, — "  biophores,"  "  gem- 
mules,"  "  micellae,"  etc.,  have  been  proposed  for 
these  assumed  units,  but  whatever  may  be  their 
structure,  they  lie  far  beyond  the  reach  of  our  best 
microscopes,  and  it  will  hardly  be  profitable  here  to 
dwell  upon  the  various  theories  that  try  to  explain 
the  ultimate  structure  of  the  protoplasm. 

Any  chemical  analysis  of  protoplasm  must  be  only 


8  Plant  Life  and  Evolution 

approximate,  since  an  essential  condition  of  active 
protoplasm  is  its  great  instability.  As  the  result  of 
the  activity  within  the  protoplasm,  new  substances 
are  constantly  being  formed,  some  being  con-/ 
structive  elements  of  the  growing  protoplasm,  others 
excretions  which  cannot  be  considered  as  truly  a 
part  of  the  living  protoplasm.  Moreover,  there  may 
be  unassimilated  food  products  from  without.  Nev- 
ertheless, in  a  general  way  the  essential  chemical 
composition  may  be  determined,  and  we  know  that 
the  protoplasm  of  all  organisms  contains  very  much 
the  same  chemical  elements.  The  chemical  elements 
which  are  absolutely  essential  are  comparatively 
few,  the  most  important  being  oxygen,  hydrogen, 
carbon,  and  nitrogen,  which  compose  the  greater 
part  of  the  protoplasm;  but  sulphur,  phosphorus, 
potassium,  calcium,  and  iron  are  never  absent  from 
normal  green  plants.  Of  course  it  must  be  assumed 
that  the  proportion  and  arrangement  of  these  are 
different  in  different  organisms. 

There  is  always  present  in  active  protoplasm  a 
large  percentage  of  water  which  is  necessary  to  put 
the  protoplasm  in  the  semi-fluid,  viscous  condition 
essential  for  its  activity.  Moreover,  the  water  is 
a  solvent  for  most  of  the  food  elements  needed  for 
the  growth  of  the  protoplasm.  The  water  thus 
serves  a  twofold  purpose;  first  as  a  mechanical 
agent  putting  the  protoplasm  in  the  labile  condition 
necessary  for  manifesting  activity;  and  second,  as 
a  means  of  transport  of  food  in  the  form  of  solu- 


Introduction  9 

tions.  Through  the  decomposition  of  water,  plants 
also  obtain  material  for  the  manufacture  of  or- 
ganic matter.  While  the  presence  of  the  water  is 
essential  for  protoplasmic  activity,  its  withdrawal 
does  not  necessarily  kill  the  protoplasm.  In  plants, 
especially,  certain  structures  like  spores  and  seeds 
may  be  deprived  as  completely  as  possible  of  all 
traces  of  free  water  without  at  all  injuring  the 
vitality  of  the  protoplasm,  which  quickly  resumes 
its  activity  when  the  necessary  water  is  supplied. 
While  it  is  true  that  the  chemical  composition  of 
all  protoplasm  is  much  the  same,  it  is  equally  clear 
that  it  cannot  be  absolutely  identical  for  any  two 
organisms.  The  enormous  complexity  of  its  con- 
stitution offers  room  for  infinite  variations  in  both 
the  composition  and  arrangement  of  the  ultimate 
protoplasmic  units;  and  it  is  reasonable  to  suppose 
that  the  inherent  variability  of  all  organisms  is  to 
be  traced  back  finally  to  these  inevitable  variations 
in  the  protoplasm  of  which  they  are  built  up. 

The  Cell. — With  few  exceptions  the  protoplasm 
is  segregated  in  definite  bodies  or  cells.  In  all  but 
the  very  lowrest  plants  the  protoplasmic  body  of  the 
cell — the  "  protoplast  " — always  contains  a  well- 
organized  nucleus  like  that  found  in  the  cells  of 
animals.  There  are  also  in  all  of  the  green  cells 
definite  bodies,  chromatophores,  in  which  is  con- 
tained the  chlorophyll.  Both  the  nucleus  and 
chromatophores  always  arise  by  division  and  are 
never  formed  anew  in  the  protoplasm.  Other 


10 


Plant  Life  and  Evolution 


bodies  may  be  sometimes  recognized,  such  as  the 
"  microsomes,"  and  there  is  every  reason  to 
look  upon  the  protoplast  as  an  organism,  made 
up  of  permanent  parts  or  organs,  of  which  only 
a  few,  like  the  chromosomes  of  the  nucleus,  and 
the  chromatophores,  are  readily  demonstrated. 


•CJ> 


FIG.  i 

A — Diagram  showing  the  parts  of  a  typical  plant  cell — w, 
cell  wall ;  cy,  cytoplasm ;  n,  nucleus ;  cr,  chromatophores ;  v, 
central  vacuole,  filled  with  watery  cell-sap. 

B — A  cell  in  process  of  division — cs,  chromosomes ;  C.P.,  be- 
ginning of  the  division-wall ;  f,  spindle-fibers. 

What  the  nature  of  the  invisible  organs — "  bio- 
phores,"  "  pangenes,"  etc. — is,  of  course,  at  the  pres- 
ent can  only  be  conjectured,  but  it  is  probable  that 
they  are  of  very  many  kinds  and  that  they  always 
multiply  by  division  as  do  the  nucleus  and  chro- 
matophores. Thus  there  are  distributed  to  the 
daughter  cells  after  each  cell  division  similar  ele- 
ments which  insure  a  great  degree  of  similarity  and 


Introduction  1 1 

determine  the  quality  of  the  cell.  As  it  is  not  con- 
ceivable that  there  should  be  an  exact  distribution 
of  the  constituents  of  the  cell  in  cell  division,  it 
necessarily  follows  that  no  two  cells  can  be  exactly 
alike,  and  these  differences  may  be  increased  by 
subsequent  changes  in  the  cells  during  the  develop- 
ment of  the  organism.  Although  it  is  probable  that 
the  invisible  units  that  make  up  the  protoplasm 
are  of  many  kinds,  it  is  also  pretty  certain  that  the 
differences  between  two  protoplasts  may  be  due  not 
so  much  to  diversity  in  the  actual  structure  of  the 
particles  of  each,  as  to  a  different  grouping  of  the 
particles,  involving  a  different  reaction  toward  the 
many  stimuli  to  which  the  protoplasmic  structures 
are  subjected  in  the  course  of  their  development. 

Plants  and  Animals. — While  all  the  higher  organ- 
isms may  be  readily  assigned  to  either  the  plant  or 
animal  kingdoms,  this  is  not  the  case  with  many 
of  the  simplest  forms  of  life,  and  the  division  of 
living  things  into  plants  and  animals  is  more  or  less 
arbitrary.  The  living  cells  of  all  organisms  are 
composed  of  protoplasm,  and  all  of  them  perform 
the  same  life  functions, — i.e.,  they  respire,  feed, 
grow,  and  reproduce,  and  these  functions  are  very 
similar  in  all  living  things.  The  differences  be- 
tween plants  and  animals  are  largely  physiological 
ones,  and  are  by  no  means  universal.  Typical 
plants — i.e.,  those  that  possess  chlorophyll — can  live 
entirely  upon  inorganic  food,  and  are  able  to  utilize 
the  radiant  energy  of  sunlight  for  the  manufacture 


12  Plant  Life  and  Evolution 

from  CO2  and  water  of  simple  organic  com- 
pounds. 

The  ability  to  feed  entirely  upon  inorganic  matter 
is  not,  however,  in  all  cases  dependent  upon  light. 
We  have  already  seen  that  certain  bacteria  can  do 
this  in  the  absence  of  light,  probably  through  the 
agency  of  some  form  of  chemical  energy.  Animals, 
so  far  as  we  know,  are  absolutely  dependent  upon 
organic  food  for  their  existence,  feeding  directly 
or  indirectly  upon  plants.  Where  chlorophyll  is 
present  in  animals,  as  for  instance  in  the  green 
hydra  and  fresh-water  sponges,  it  has  been  shown 
that  the  green  color  is  due  to  the  presence  of  minute 
green  algae  which  live  within  the  tissues  of  their 
host.  Of  unicellular  animals,  it  has  been  claimed 
that  a  species  of  Vorticella  contains  chlorophyll,  and 
a  common  flagellate  organism  Euglena,  which  is 
structurally  more  like  an  animal  than  a  plant,  and 
is  often  considered  to  be  a  true  animal,  contains 
abundant  chlorophyll  and  is  undoubtedly  capable  of 
photosynthesis. 

Plants  the  Manufacturers  of  Organic  Matter. — 
Plants  being  the  manufacturers  of  all  organic  food, 
their  importance  in  the  economy  of  nature  is  at 
once  evident.  Without  them  all  animal  life  would 
necessarily  soon  cease.  While  the  green  plants  take 
the  first  place  in  the  manufacture  of  organic  com- 
pounds, it  must  be  remembered  that  the  lowest  of 
all  plants,  the  bacteria,  are  also  indispensable  in 
maintaining  the  circulation  of  materials  necessary 


Introduction  13 

for  the  nutrition  of  all  higher  organisms.  This 
is  accomplished  in  two  ways;  first,  by  the  assimila- 
tion of  free  nitrogen  and  the  changing  of  certain 
nitrogeneous  constituents  of  the  soil  into  forms  that 
can  be  used  by  the  higher  plants;  and  second, 
through  the  decomposition  of  dead  organic  matter 
which  is  thus  put  in  such  a  form  that  it  can  again 
be  used  by  green  plants.  All  organisms  in  the 
course  of  their  development  give  off  waste  products 
which  ultimately  serve  as  food  for  other  organisms. 
Most  of  these  waste  products  except  CO2  and  water 
are  not  directly  available  for  the  food  of  green 
plants,  but  must  first  be  acted  upon  by  bacteria. 
The  bacteria,  also,  as  has  been  indicated,  are  the 
principal  agents  by  which  the  dead  tissues  of  plants 
and  animals  are  decomposed  so  that  they  are  again 
available  as  food  for  the  higher  plants.  There 
are,  however,  many  undoubted  plants  such  as  the 
fungi — i.e.,  molds,  mildews,  rusts,  mushrooms,  etc. 
— as  well  as  certain  flowering  plants  that  are  desti- 
tute of  chlorophyll,  like  the  dodder  and  Indian  pipe, 
which  require  organic  food  just  as  animals  do. 

We  may  say,  however,  that  the  typical  plant  con- 
tains chlorophyll  and  has  the  power  of  photosyn- 
thesis; that  is,  can  use  the  energy  of  sunlight  for 
the  manufacture  of  organic  compounds  from  CO2 
and  water.  Animals  are  not  provided  with  chloro- 
phyll and  are  entirely  dependent  upon  organic  food 
for  their  existence.  Animals  are  as  a  rule  much 
more  mobile  than  are  plants,  and  this  is  correlated 


14  Plant  Life  and  Evolution 

with  the  different  method  of  th^ir  nutrition.  With 
few  exceptions  the  cells  of  plants  are  surrounded  by 
a  firm  membrane  which  precludes  motion  except 
where  there  are  openings  for  the  protrusion  of  the 
protoplasm,  a  condition  which  sometimes  occurs  in 
unicellular  plants  or  those  composed  of  a  small  num- 
ber of  cells.  The  tissues  of  the  higher  plants,  made 
up  of  these  firm  walled  cells,  are  capable  of  only  a 
limited  amount  of  motion.  In  animals  the  cell-wall 
is  much  less  evident  in  active  tissues,  and  is  often 
not  developed  at  all,  thus  allowing  a  much  greater 
degree  of  mobility  in  the  cells  than  is  the  case  in 
plant  tissues. 

Plants  Immobile  Organisms. — The  plant  having 
a  constantly  renewed  food  supply,  CO2  from  the 
atmosphere,  and  water  and  mineral  compounds  from 
the  earth,  has  no  need  to  move  from  its  position; 
while  the  animal,  obliged  to  move  from  place  to 
place  in  search  of  food,  must  be  provided  with 
special  organs  of  locomotion.  If,  however,  an  ani- 
mal is  so  placed  that  it  is  provided  with  a  constant 
supply  of  food  within  its  reach,  it  may  often  show  a 
plant-like  immobility.  This  is  seen  in  the  case  of 
many  parasites,  and  in  such  aquatic  animals  as 
sponges,  hydroids,  and  corals,  which  in  their  adult 
condition  sometimes  curiously  mimic  vegetable 
forms  very  closely,  hence  their  old  name  of  "  zo- 
ophytes." Many  mollusks,  like  the  oyster  and  mus- 
sel, are  also  fixed  in  their  adult  condition.  Such 
stationary  animals  have  developed  ciliated  organs 


Introduction  15 

which  create  currents  in  the  water,  carrying  with 
them  the  organisms  needed  for  food.  These  fixed 
animals,  at  some  stage  in  their  development,  must 
provide  for  dispersing  themselves,  and  very  com- 
monly the  larvae  are  actively  motile.  In  other  cases 
it  is  the  adult  which  is  active,  as  with  many  insect 
parasites,  the  larvae  being  incapable  of  moving  away 
from  their  host.  The  same  necessity  for  the  dis- 
tribution of  the  species  is  seen  in  plants,  whose 
reproductive  parts  show  many  methods  of  dispersal. 
In  the  lower  plants,  freely  motile  reproductive  cells 
are  common.  In  the  higher  plants,  the  distribution 
of  the  reproductive  parts — spores,  seeds,  etc. — is 
usually  passive,  but  these  structures  are  often  modi- 
fied so  as  to  facilitate  the  distribution  by  special 
means,  such  as  dispersal  by  the  wind  of  many 
winged  seeds  and  fruits,  or  the  development  of 
hooks  by  which  these  adhere  to  animals  and  are 
thus  transported. 

The  fixed  position  of  plants  involves  a  high  de- 
gree of  adaptability,  and  they  show  a  capacity  for 
growth  and  regeneration  that  can  hardly  be  matched 
in  animals.  The  most  highly  specialized  plants  are 
far  less  individualized  than  the  majority  of  ani- 
mals ;  indeed  it  is  not  always  easy  to  limit  the  indi- 
vidual, as  most  plants  may  perhaps  be  looked  upon 
rather  as  a  colony  of  united  units  than  as  a  single 
individual.  Not  only  are  the  cells  of  plants  less 
various  than  those  of  animals,  but  there  may  be  also 
an  almost  unlimited  repetition  of  similar  organs. 


16  Plant  Life  and  Evolution 

We  have  but  to  compare  a  tree  like  the  oak,  for 
example,  with  a  highly  specialized  animal  like  the 
horse.  The  former  may  live  for  centuries,  produc- 
ing each  year  thousands  of  leafy  twigs  and  flowers, 
each  like  its  neighbors.  The  complexity  of  the  plant 
is  due  to  the  multiplication  of  similar  organs  rather 
than  to  a  great  number  of  different  parts.  The 
animal  is  sharply  individualized,  its  different  organs 
being  absolutely  definite  in  number  and  position 
with  only  a  limited  power  of  regeneration.  It  has 
a  very  limited  and  definite  period  of  growth,  with 
a  correspondingly  brief  life-span. 

Regeneration  in  Plants. — The  power  of  regenera- 
tion in  the  higher  animals  is  very  limited.  A 
wound  may  heal,  and  such  organs  as  nails,  feathers, 
and  hair  may  be  replaced,  and  in  some  lower  verte- 
brates even  a  whole  limb  or  tail  may  be  regenerated. 
But  in  the  higher  forms  of  animals,  the  regenera- 
tion of  whole  individuals,  except  sexually,  never 
occurs.  In  plants,  on  the  other  hand,  even  in  the 
highest  ones,  the  regeneration  of  the  whole  indi- 
vidual from  almost  any  member  of  the  body  is 
possible,  and  this  fact  is  constantly  taken  advan- 
tage of  in  the  artificial  propagation  of  plants  by 
cuttings,  grafts,  etc.  In  some  cases  a  fragment  of 
a  leaf  or  root  is  enough  for  the  development  of  a 
complete  plant,  and  in  many  of  the  lower  plants, 
a  single  cell  is  sufficient.  Plants  also  may  show  a 
regular  periodic  regeneration  of  certain  organs,  such 
as  leaves  and  flowers,  which  are  short-lived  and 


Introduction  17 

perish  after  they  have  served  their  purpose  and  are 
replaced  each  season  by  entirely  new  formations. 
While  this  development  of  temporary  organs  may 
occur  in  animals,  i.e.,  the  moulting  of  special  plumes 
in  the  breeding  season  of  birds,  the  seasonal  de- 
velopment of  the  horns  of  deer,  the  summer  and 
winter  coats  of  many  animals,  etc.,  these  organs 
are  not  essential  ones,  and  the  structures  in  which 
they  develop  are  permanent,  as  for  instance  the 
feather  and  the  hair-papillse. 

Plants  Less  Specialized  than  Animals. — There 
is,  in  short,  far  less  difference  between  the  higher 
and  lower  forms  among  plants  than  is  the  case 
among  animals.  The  relative  simplicity  of  even 
the  highest  plants  perhaps  accounts  for  their  greater 
plasticity.  Their  immobile  condition  makes  it  nec- 
essary for  them  to  be  able  to  endure  the  changes  of 
temperature,  light,  moisture  to  which  they  may  be 
subjected,  as  they  have  no  power  to  move  away  in 
search  of  more  favorable  conditions  after  they  once 
become  fixed.  Many  plants  show  an  extraordinary 
adaptability  for  growing  under  extremely  diverse 
conditions,  and  may  be  so  changed  as  to  be  scarcely 
recognized  as  the  same  species.  Thus  a  tree  which 
grows  to  a  large  size  in  a  sheltered  valley,  may  sur- 
vive near  the  timber  line  on  a  lofty  mountain,  as 
a  prostrate  shrub,  rising  but  a  few  inches  from  the 
ground ;  or  a  weed,  growing  tall  and  rank  in  a  damp 
and  shady  fence  corner,  may  shrink  to  a  tiny  herb 
with  minute  leaves  and  thin  wiry  stems,  when  grow- 


1 8  Plant  Life  and  Evolution 

ing  in  a  sun-baked,  hard-trodden  path ;  and  yet  they 
will  succeed  in  flowering  and  maturing  their  seeds 
under  these  adverse  conditions.  This  adaptability 
of  plants  and  the  readiness  with  which  they  respond 
to  changes  in  environment  make  them  especially 
suited  to  experiments,  and  in  many  ways  they  are 
therefore  easier  to  study  than  are  most  animals.  It 
is  not  strange  then  that  they  should  have  been  made 
the  subject  of  many  experiments  bearing  on  the 
question  of  the  factors  concerned  in  organic  evolu- 
tion and  the  origin  of  species. 


CHAPTER  II 
FACTORS  IN  EVOLUTION 

PROFESSOR  H.  F.  OSBORN  has  recently 
enunciated  the  law  of  four  inseparable  factors 
that  may  be  denominated  the  primary  processes  of 
evolution.  He  says  :  "  The  life  and  evolution  of  or- 
ganisms continuously  center  around  the  processes 
which  we  term  heredity,  ontogeny,  environment, 
and  selection.  These  have  been  inseparable  and  in- 
teracting from  the  beginning;  a  change  introduced 
'or  initiated  through  any  one  of  these  factors  causes 
a  change  in  all." 

HEREDITY 

Heredity. — The  transmission  of  like  character- 
istics from  parent  to  offspring  is  a  sufficiently 
familiar  phenomenon,  but  the  factors  directly  con- 
cerned with  this  are  by  no  means  clearly  under- 
stood. It  is  inconceivable  that  under  any  circum- 
stances an  acorn  should  produce  anything  but  an 
oak,  or  that  the  offspring  of  a  dog  should  be  any- 
thing but  a  dog;  but  why  the  germ  cell  of  a  specific 
organism  should  always  follow  the  same  course  of 
19 


2O  Plant  Life  and  Evolution 

development  is  not  so  obvious.  In  the  majority  of 
animals  new  individuals  can  arise  only  from  the 
specialized  reproductive  cells,  and  as  a  rule  the 
formation  of  a  new  individual  is  the  result  of  the 
fusion  of  two  gametes,  or  sexual  cells,  the  ovum 
and  spermatozoon,  which  are  markedly  different 
from  the  somatic  cells,  i.e.,  the  cells  that  compose 
the  various  tissues  of  the  body.  In  very  many 
plants  there  is  a  similar  development  of  sexual  cells, 
and  the  process  of  fertilization  is  essentially  the 
same  as  in  animals;  but  most  plants  also  multiply 
asexually,  and  not  a  few  plants  are  known  in  which 
this  is  the  only  method  of  propagation.  Hence  we 
must  remember  that  hereditary  characteristics  are 
not  transmitted  by  sexual  cells  alone.  The  plants 
whose  propagation  is  strictly  asexual  are  not  only 
the  lowest  forms,  like  the  bacteria,  but  also  a  good 
many  algse  and  fungi,  some  of  them  plants  of  large 
size  and  complex  structure.  Among  cultivated 
plants  are  many  which  rarely  or  never  produce 
perfect  seeds  and  are  always  propagated  by  division. 
The  banana,  pineapple,  breadfruit,  and  sugar  cane 
are  examples  of  these,  and  the  many  varieties  of 
domesticated  plants  which  have  arisen  in  cultiva- 
tion transmit  their  characters  in  a  purely  non- 
sexual  way.  Bud  variation  is  a  not  infrequent 
phenomenon,  and  such  variations  are  readily 
perpetuated  by  cuttings  or  grafts. 

Asexual  Reproduction  in  Plants. — No  cases  are 
known  among  the  vascular  plants,  i.e.,  ferns  and 


Factors  in  Evolution  21 

seed  plants,  where  a  single  somatic  cell  can  develop 
into  a  new  plant;  but  in  some  liverworts  this  is 
possible,  and  among  the  algae  it  is  a  common  phe- 
nomenon. These  asexual  reproductive  cells  in  the 
latter  plants  are  often  free  swimming  "  zoospores." 
This  great  regenerative  power  shown  by  the  vege- 
tative cells  of  plants  is  entirely  in  harmony  with 
the  generally  lower  degree  of  specialization  shown 
by  plants  when  compared  with  animals. 

The  capacity  of  reproducing  from  a  single  cell, 
the  egg,  the  whole  of  an  exceedingly  complex  or- 
ganism, shown  especially  in  the  case  of  the  higher 
animals,  has  led  to  much  speculation  concerning  the 
actual  structures  that  are  the  basis  of  hereditary 
transmission.  The  theory  of  a  special  "  germ- 
plasm,"  "  biophores,"  "  pangenes,"  etc.,  which  are 
the  bearers  of  hereditary  characters,  has  been  the 
subject  of  many  ingenious  speculations,  none  of 
which,  however,  is  capable  of  actual  demonstration. 
The  phenomena  of  nuclear  division  point  to  the 
chromosomes  as  being  important  agents  in  heredity, 
but  it  is  not  likely  that  they  are  the  sole  bearers 
of  hereditary  characters. 

The  Agents  in  Heredity. — The  immensely  com- 
plex structure  of  the  protoplast  permits  of  infinite 
variation  in  the  arrangement  of  its  constituents,  and 
whether  or  not  we  assume  the  presence  of  special 
permanent  determining  structural  units  such  as  the 
"  determinants  "  of  Weismann,  it  is  evident  that  the 
germ  cells  of  every  organism  possess  their  own  in- 


22  Plant  Life  and  Evolution 

dividual  characters,  and  these  characters  must  be 
transmitted  through  cell  division  to  all  of  the  de- 
scendants of  the  germ  cell.  The  ultimate  structure 
of  the  germ  cells  of  two  species  being  different,  it 
follows  that  their  responses  to  the  various  stimuli 
to  which  they  are  subjected  during  the  develop- 
ment of  the  embryo  will  also  differ,  and  moreover 
the  conditions  to  which  the  developing  embryos  of 
any  two  species  are  exposed  also  may  be  supposed 
to  differ  to  a  greater  or  less  degree. 

In  a  general  way  we  may  say  that  the  degree 
of  difference  between  two  organisms  is  deter- 
mined, first,  by  structural  differences  of  the  germ 
cells,  and  secondly  by  the  different  conditions  to 
which  the  germ  cell  is  exposed  in  the  course  of 
its  normal  development.  Given  two  identical 
germ  cells,  subject  to  identical  conditions  through- 
out their  development,  and  the  result  must  be 
two  identical  organisms.  Pfeffer  very  properly 
lays  stress  upon  the  importance  of  the  physi- 
ological factors  in  heredity  as  distinguished  from 
purely  structural  ones,  and  this  view  has  also 
been  expounded  by  Peirce,  Farmer,  and  other 
physiologists.  The  chemical  and  physical  stimuli 
to  which  the  protoplasmic  units  are  constantly  sub- 
jected are  quite  as  potent  in  determining  the  char- 
acter of  the  resulting  structure  as  is  the  mere  chem- 
ical composition  of  the  different  protoplasmic  units 
of  which  the  germ  cell  is  composed.  It  does  not, 
however,  follow  that  the  offspring  must  be  an  exact 


Factors  in  Evolution  23 

duplicate  of  the  parent,  and  we  know  that  it  always 
departs  more  or  less  from  the  parental  type.  These 
departures  from  the  type  may  be  very  marked, 
and  it  is  not  unlikely  that  these  changes  may  be 
so  great  sometimes  as  to  pass  beyond  the 
limits  of  the  so-called  fluctuating  variations  that 
are  common  to  all  species,  and  in  such  cases 
there  arise  so-called  "  mutations,"  which  may 
be  permanent  in  case  crossing  is  prevented.  Such 
sudden  or  discontinuous  variations  are  assumed  by 
some  biologists  to  be  the  all-important  cause  of  the 
formation  of  new  species.  This  view  is  especially 
held  by  De  Vries,  whose  studies  in  mutation  have 
lately  attracted  so  much  attention.  As  to  the  causes 
of  these  mutations,  however,  we  are  very  much  in 
the  dark,  and  much  more  evidence  is  needed  before 
it  will  be  safe  to  assume  that  mutations  alone  are 
the  real  origins  of  new  species. 

That  the  constitution  of  all  the  germ  cells  of  a 
given  species  is  essentially  the  same,  and  that  in  the 
normal  course  of  development  the  growing  organism 
is  subject  to  the  same  conditions,  will  account  for 
the  main  facts  of  heredity,  without  assuming  any 
special  germ-plasm  or  "  formative  materials " 
corresponding  to  special  organs.  The  phenomena 
of  regeneration  in  plants  all  point  to  the  correctness 
of  this  view. 

There  must  be  inevitably  a  greater  or  less  differ- 
ence between  the  cells  resulting  from  any  cell  di- 
vision, and  these  differences  must  be  reflected  in  the 


24  Plant  Life  and  Evolution 

individual  differences  existing  between  any  two 
members  of  the  same  species.  The  germ  cells,  as  a 
rule,  are  so  situated  as  to  be  less  subject  to  external 
influences,  and  are  much  more  stable  than  the  ordi- 
nary vegetative  or  somatic  cells.  How  far  they  may 
be  affected  by  external  conditions,  and  to  what  ex- 
tent, if  any,  changes  thus  affected  are  transmitted 
by  heredity,  is  one  of  the  questions  which  has  been 
very  much  discussed,  but  about  which  there  is  really 
very  little  positive  evidence. 

ONTOGENY 

Ontogenetic  Variations. — Every  organism  passes 
through  a  more  or  less  extensive  series  of  changes 
during  its  development  from  the  germ  to  maturity. 
In  the  course  of  its  life-history,  or  "  ontogeny,"  in- 
dividual variations  occur,  some  of  which  can  be 
attributed  to  the  environment,  while  others  are  ap- 
parently innate.  It  is  these  ontogenetic  variations 
which  distinguish  the  innumerable  individuals  be- 
longing to  a  species.  We  may  examine  a  hundred 
seedlings,  reared  apparently  under  the  same  condi- 
tions, and  no  two  will  be  exactly  alike.  The  biolo- 
gist has  no  more  difficult  problem  than  that  of 
determining  the  causes  of  these  variations.  While 
in  many  cases  the  effects  of  extrinsic  factors  can 
be  easily  demonstrated,  as  for  instance  dwarfing 
caused  by  insufficient  nutrition,  more  often  the  varia- 
tions seem  to  be  innate  differences  which  begin  in 


Factors  in  Evolution  25 

the  germ,  and  are  due  to  more  or  less  marked  diver- 
gencies in  the  very  constitution  of  the  germ  cells, 
and  these  differences  are  reflected  in  the  organisms 
developed  from  them.  How  far  these  ontogenetic 
variations  are  transmissible  is  hard  to  determine, 
but  it  seems  reasonable  to  suppose  that  they  are 
not  without  their  effect  in  determining  the  future 
history  of  the  race. 

ENVIRONMENT 

Irritability. — Protoplasm  is  distinguished  by  the 
remarkable  property  of  irritability,  i.e.,  the  power 
of  reacting  to  various  external  stimuli,  such  as  light, 
heat,  electric  currents,  mechanical  shocks,  etc.,  as 
well  as  to  the  so-called  automatic  stimuli,  or  those 
arising  within  the  protoplasm  itself.  These  external 
stimuli  constitute  an  important  part  of  the  environ- 
ment which  is  so  powerful  a  factor  in  the  shaping 
of  every  organism.  Were  the  protoplasm  abso- 
lutely uniform  in  all  cases  and  the  environment  con- 
stant, there  would  necessarily  be  no  change,  and 
evolution  could  obviously  not  proceed.  But  by  their 
very  nature  the  protoplasts  of  no  two  cells  can  be 
exactly  alike  in  structure,  and  there  must  be  a  cor- 
responding degree  of  variability  also  in  their  be- 
havior towards  any  stimuli.  Moreover,  the  environ- 
ment cannot  remain  absolutely  constant  but  must 
change  to  a  greater  or  less  degree.  Inherent  varia- 
bility in  the  structure  of  the  protoplast,  and  the 


26  Plant  Life  and  Evolution 

inevitable  fluctuations  in  the  environment  may  be 
considered  as  the  fundamental  causes  of  that  varia- 
tion which  is  the  beginning  of  any  line  of  evolution. 
The  Effects  of  Stimuli. — As  we  are  very  ignorant 
of  the  physical  structure  of  protoplasm  we  can  only 
guess  at  the  reactions  that  are  developed  within  it 
as  the  results  of  various  stimuli.  While  we  speak 
of  the  formative  effects  of  light,  heat,  and  other 
extrinsic  factors,  it  is  extremely  unlikely  that  the  ef- 
fects of  these  are  immediate.  As  the  results  of  their 
action  certain  effects  finally  develop;  but  how  far 
these  are  the  direct  result  of  the  evident  stimuli, 
and  how  far  they  are  caused  by  others  not  so  ap- 
parent, we  have  no  means  of  judging.  It  is  certain 
that  a  single,  apparently  insignificant  stimulus  may 
set  in  motion  a  chain  of  reactions  which  result  in 
far-reaching  effects.  We  might  compare  this  to  a 
mine,  which  may  be  fired  by  a  single  spark,  or  by 
a  percussion  cap,  the  final  result  of  the  explosion 
being  the  annihilation  of  a  whole  town.  A  fern- 
spore  lies  dormant  in  a  state  of  physiological  equi- 
librium ;  it  is  placed  in  water,  and  immediately  there 
is  set  up  a  series  of  reactions  which  result  in  its 
germination  and  final  development  into  the  mature 
plant.  Light  or  heat  may  be  the  stimulus  which 
is  necessary,  but  once  inaugurated  the  succession  of 
reactions  must  follow.  The  fertilization  of  the  egg 
and  its  subsequent  development  into  the  animal  of- 
fers an  equally  striking  instance  of  the  far-reaching 
results  of  an  apparently  slight  stimulus. 


Factors  in  Evolution  27 

Cumulative  Effects  of  Stimuli. — It  has  often  been 
demonstrated  that  the  effects  of  stimuli  may  be 
cumulative,  and  that  when  a  stimulus  is  repeated  at 
short  intervals  the  response  to  this  may  be  very 
different  in  the  later  cases  of  stimulation.  Thus 
Jennings,  in  his  important  studies  on  the  reactions 
of  the  Infusoria,  has  shown  that  they  may  become 
habituated  to  a  stimulus,  and  fail  to  respond  to  it 
again  after  it  has  been  repeated  several  times.  It 
would  appear  that  the  "  physiological  state  "  of  the 
cell  has  changed  as  the  result  of  the  stimulus,  and 
in  a  very  suggestive  recent  address  by  Prof.  F. 
Darwin,  he  brings  forward  the  theory  that  this 
permanence  of  the  effects  of  stimulation  upon  the 
protoplasm,  or  "  memory  "  as  he  boldly  puts  it,  is 
perhaps  the  most  potent  of  all  causes  in  determining 
the  course  of  evolution  in  the  development  of  an 
organism. 

Experiments  with  Slime-molds. — Where  proto- 
plasm occurs  in  large  masses,  as  it  does  in  those 
remarkable  organisms  the  slime-molds,  its  reaction 
to  various  stimuli  is  easily  demonstrated.  Exposed 
to  strong  light,  the  slimy  mass  or  "  plasmodium  " 
will  seek  shelter  in  the  crevices  of  the  rotten  log  on 
which  it  is  growing  or  will  hide  under  the  masses  of 
dead  leaves  or  tan  bark  which  may  serve  it  for 
food.  The  movements  will  be  accelerated  by  an 
increase  of  temperature ;  withdrawal  of  moisture 
will  cause  a  contraction  or  even  make  it  assume  a 
quiescent  stage;  and  in  short  the  sensitive  mass  of 


28  Plant  Life  and  Evolution 

living  slime  responds  promptly  to  the  various  stimuli 
which  may  be  brought  to  bear  upon  it. 

As  a  rule,  however,  the  protoplasm  of  plants  is 
shut  up  in  closed  cells,  but  the  living  protoplast 
included  within  the  cells  often  shows  evident  move- 
ments, and  may  be  seen  to  react  toward  stimuli  in 
the  same  way  as  the  naked  plasmodium  of  the 
slime-mold.  If  the  green  corpuscles  or  chloroplasts 
are  present  in  the  cells,  these  may  be  seen  to  shift 
their  position  under  the  influence  of  light,  and  the 
protoplasmic  movements  which  are  very  common 
within  the  cells  are  affected  readily  by  different 
stimuli.  When  the  protoplasm  escapes  from  the 
cell,  as  it  sometimes  does  in  the  reproductive  cells, 
especially  among  the  lower  plants,  these  motile  cells 
usually  react  promptly  to  various  stimuli.  Thus 
zoospores  will  usually  swim  towards  the  source  of 
light,  and  the  spermatozoids  of  ferns  are  strongly 
attracted  by  the  salts/ of  malic  acid. 

Reactions  of  Multicellular  Organs,  Due  to  Irri- 
tability of  Protoplasm. — The  movements  and  other 
indications  of  response  to  stimuli,  shown  by  the 
multicellular  organs  of  the  higher  plants,  are  un- 
doubtedly induced  primarily  by  the  reaction  of  the 
protoplasm  within  their  cells.  It  has  been  shown 
that  in  many  plants  there  is  a  direct  communication 
between  the  protoplasts  of  neighboring  cells,  due 
to  the  penetration  of  the  cell  walls  by  the  fine 
threads  of  protoplasm,  and  it  is  highly  probable  that 
in  this  way  the  effects  of  stimuli  may  be  propagated 


Factors  in  Evolution  29 

from  cell  to  cell,  somewhat  as  is  the  case  in  animals, 
where  there  is  a  specialized  nervous  system.  Our 
knowledge  of  the  transmission  of  stimuli  in  plants, 
however,  is  very  far  from  complete.  Of  a  very  dif- 
ferent nature  are  certain  movements  which  are 
purely  mechanical.  Thus  the  twisting  of  the  awns 
in  many  grasses,  or  in  the  alfilaria  (Erodium),  or 
the  movements  of  the  "  elaters  "  in  Equisetum  or 
the  liverworts,  are  purely  mechanical  movements 
due  to  the  absorption  of  water. 

Unicellular  Plants. — The  simplest  green  plants 
consist  of  a  single  cell,  which  may  be  motile,  but 
usually  is  non-motile  and  most  often  globular  or 
oval  in  form.  The  cell  is  usually  enclosed  in  a 
membrane  of  cellulose.  The  protoplast  contains  a 
nucleus  and  one  or  more  green  corpuscles  or 
chromatophores.  Such  a  simple  green  cell  is  able 
to  perform  all  the  essential  life  functions.  These 
low  plants  are  mostly  aquatic,  and  with  the  water 
absorbed  from  the  medium  surrounding  them,  there 
are  taken  into  the  cell  in  solution  the  various  food 
constituents  which  the  cell  needs  for  its  develop- 
ment. Oxygen  is  absorbed  for  the  respiratory  proc- 
ess, and  in  the  chromatophores  the  CO2  dissolved 
in  the  water  is  decomposed  and  united  with  the 
hydrogen  and  oxygen  derived  from  the  decomposi- 
tion of  water.  Such  green  cells  exposed  to  the  light 
for  a  short  time  will  show  in  the  chromatophores 
the  first  visible  evidence  of  their  assimilation,  or 
photosynthesis,  in  the  form  of  starch,  this  carbon 


30  Plant  Life  and  Evolution 

assimilation  being  accompanied  by  the  evolution  of 
free  oxygen.  The  cell  increases  in  size  until  it 
reaches  its  full  development  and  then  by  division 
two  cells  are  formed,  which  constitute  new  indi- 
viduals. 

Such  simple  unicellular  plants  react  promptly  to 
the  environment.  Deprived  of  light,  photosynthesis 
at  once  ceases;  changes  in  temperature  materially 
effect  the  activity  of  nutrition  and  growth  which 
only  can  be  maintained  within  certain  often  decid- 
edly small  limits;  should  the  temperature  of  the 
water  in  which  the  plant  is  growing  be  raised  above 
a  certain  point,  death  will  result.  Most  of  the  lower 
plants  are  much  more  resistant  to  low  temperatures 
and  may  often  be  frozen  solid  without  injury. 
Withdrawal  of  water  does  not  necessarily  destroy 
the  plant.  Many  of  them  simply  become  dormant, 
remaining  inert  so  long  as  they  are  dry,  but  absorb- 
ing water  quickly  when  moistened,  and  soon  resum- 
ing activity.  Retention  of  moisture  is  often  facili- 
tated by  the  gelatinous  character  of  their  walls, 
which  hold  water  very  tenaciously. 

Most  Plants  are  Multicellular. — While  there  are 
many  plants  which  are  unicellular  and  some  of  these 
are  quite  highly  specialized,  the  greater  number  of 
plants  are  multicellular.  The  simplest  multicellular 
plants  are  filamentous  algae,  rows  of  often  slightly 
coherent  and  quite  uniform  cells.  Such  a  plant 
might  be  considered  as  a  chain  of  unicellular  indi- 
viduals rather  than  a  multicellular  individual,  and 


Factors  in  Evolution  31 

scmetimes  a  single  cell  may  separate  and  give  rise 
immediately  to  a  new  filament.  Many  of  these 
lower  filamentous  algae  may  under  certain  condi- 
tions assume  a  unicellular  condition  and  this  may 
be  induced  artificially,  this  unicellular  stage  being 
very  marked  when  the  plants  are  grown  in  concen- 
trated culture  solutions  of  high  osmotic  pressure. 

Somewhat  higher  in  the  scale  are  forms  that 
show  polarity,  this  polarity  being  already  deter- 
mined in  the  free  swimming  spore  from  which  the 
plant  arises.  The  forward  end  of  the  spore  at- 
taches itself,  and  probably  in  response  to  the  contact 
stimulus,  develops  a  root-like  organ.  There  is  thus 
a  certain  degree  of  specialization  shown  in  the  dif- 
ferent parts  of  the  plant  and  this  becomes  more 
pronounced  in  the  more  highly  specialized  algae. 
While  in  the  simplest  forms  the  cells  are  nearly 
alike,  in  the  larger  and  more  complex  types  there 
is  a  greater  or  less  degree  of  specialization  of  the 
tissues  and  organs  more  or  less  directly  correlated 
with  the  responses  to  various  stimuli.  For  exam- 
ple, organs  resembling  the  leaves  of  the  higher 
plants  occur  in  some  algse,  these  leaf-like  structures 
being  evidently  organs  especially  adapted  to  the 
work  of  photosynthesis.  There  also  may  be  modi- 
fications in  these  large  algse  of  the  tissues  asso- 
ciated with  protection  against  loss  of  water,  for 
conduction,  storage,  etc.,  or  adaptations  enabling  the 
plant  to  withstand  the  beating  of  the  surf. 

From  these  humble  beginnings  have  gradually  de- 


32  Plant  Life  and  Evolution 

veloped  all  the  myriad  forms  of  plant  life  that  now 
exist  upon  the  earth. 

Are  Ontogenetic  Variations  Inherited? — Differ- 
ent individuals  of  the  same  species  in  the  course 
of  their  development  may  be  subject  to  very  different 
conditions,  and  these  differences  are  reflected  in  the 
change  of  form  or  in  structural  modifications  that 
are  often  very  marked  indeed.  That  such  changes 
arising  in  the  course  of  ontogeny  may  assume 
hereditary  characters  has  not  been  positively  proved, 
but  it  is  quite  probable  that  where  changed  environ- 
mental conditions  exist  for  very  long  periods  of 
time,  the  ontogenetic  structural  changes  might  be 
fixed,  thus  assuming  hereditary  value.  Where,  for 
instance,  plants  from  cold  countries  are  grown  in 
milder  ones,  their  habits  may  be  much  altered. 
Trees  which  are  deciduous  in  cold  climates  may  hold 
their  leaves  much  longer  or  even  assume  an  ever- 
green habit  when  transferred  to  a  warmer  coun- 
try. The  time  of  flowering  is  also  much  influenced 
by  climate.  For  instance,  in  the  mild,  even  tem- 
perature of  the  coast  region  of  California,  many 
garden  flowers  which  in  the  Eastern  States  have 
a  marked  seasonal  growth  and  flower  at  a  definite 
time,  may  flower  almost  at  any  time  of  the  year, 
depending  mainly  upon  the  amount  of  water  given 
them,  water  being  the  principal  factor  in  plant 
growth  in  this  region  where  it  is  never  cold  enough 
to  entirely  stop  growth.  Many  spring  flowering 
plants,  such  as  the  iris,  primrose,  violets,  and  many 


Factors  in  Evolution  33 

others  often  begin  to  flower  in  the  autumn  and  con- 
tinue flowering  for  several  months,  and  autumn  flow- 
ering plants  like  the  chrysanthemum  may  prolong 
their  flowering  period  until  February  or  March. 

How  far  these  changes  are  intensified  by  long 
cultivation  in  a  new  environment,  has  not  been  criti- 
cally studied,  and  of  course  if  the  plants  were  re- 
stored to  the  colder  climates  from  which  they  came, 
they  would  have  to  revert  to  their  original  habits 
or  perish,  as  their  changed  habits  of  growth  would 
be  quite  impossible  in  their  original  habitat. 

THE  CONDITIONS  FOR  PLANT  GROWTH 

Of  the  external  factors  that  govern  the  life  of 
normal  green  plants  the  most  important  are  light, 
heat,  gravity,  water,  oxygen,  and  various  food  con- 
stituents, including  CO2. 

Light. — As  all  green  plants  are  dependent  upon 
light  for  their  existence,  it  is  quite  comprehensible 
that  light  has  evidently  exercised  a  very  powerful 
formative  action  upon  plant  structures.  The  form 
of  each  individual  plant  is  markedly  influenced  by 
the  character  of  the  light  to  which  it  is  exposed; 
a  fact  readily  verified  by  the  most  casual  observa- 
tion. The  bending  of  plants  towards  the  light,  the 
weak  spindling  growth  where  the  light  is  insufficient, 
and  the  complete  suppression  of  chlorophyll  and 
the  reduction  in  the  size  of  the  leaves,  which  are 
such  common  phenomena  where  light  is  excluded, 


34  Plant  Life  and  Evolution 

are  familiar  to  every  one.  The  bleached  potato 
sprouts  in  a  cellar,  or  the  yellow  blades  of  grass 
under  a  board,  are  sufficiently  striking  examples  of 
the  effect  of  the  exclusion  of  light.  As  might  be 
expected,  the  parts  most  conspicuously  affected  are 
those  directly  associated  with  photosynthesis.  This 
is  seen  in  the  great  reduction  of  the  leaf  surface  and 
the  absence  of  chlorophyll  in  most  of  the  higher 
plants  when  they  are  grown  in  darkness. 

Reaction  of  Unicellular  Plants  to  Light. — The 
modifications  of  the  plant  body  associated  with  light 
adaptation  are  by  no  means  confined  to  the  higher 
plants.  Unicellular  plants  often  react  very  promptly 
to  light,  moving  toward  the  light  when  they  are 
motile  or  shifting  the  position  of  their  chromato- 
phores  according  to  the  direction  and  intensity  of 
the  light  rays.  A  similar  shifting  of  the  chromato- 
phores  in  response  to  light  may  be  demonstrated 
also  in  the  cells  of  the  higher  plants.  The  form  of 
the  chromatophores  also  may  be  explained  as  a  case 
of  light  adaptation.  They  are  most  commonly  flat- 
tened discs  or  thin  plates  of  various  forms,  thus 
exposing  large  surfaces  to  the  light  rays. 

Photosynthetic  Organs  of  the  Lower  Plants — 
Low  down  in  the  scale  of  plant  life  there  is  evidence 
of  the  development  of  special  structures  associated 
with  photosynthesis.  Two  types  of  photosynthetic 
adaptation  are  met  with  among  the  lower  algae.  In 
some  of  these,  like  the  sea-lettuce  (Ulva),  the  whole 
plant  has  the  form  of  a  thin  lamina  or  thallus,  thus 


Factors  in  Evolution  35 

displaying  a  large  surface  of  green  tissue  to  the 
action  of  light;  in  other  algae  there  may  be  dense 
tufts  of  small  branches  whose  cells  have  large 
chromatophores.  In  both  of  these  cases  there  is 
evidently  an  increase  in  the  number  of  green  cells 


FIG.  2 

Development  of  similar  leaf-like  photosynthetic  organs  in 
two  quite  unrelated  plants. 
A — A  moss  (Tetraphis). 
B — An  alga  (Sargassum). 

with  a  corresponding  increase  in  the  capacity  for 
carbon  assimilation. 

In  some  of  the  highly  organized  seaweeds,  like 
the  kelps,  there  are  definite  leaf-like  organs  at- 
tached to  the  stem  or  axis  of  the  plant.  This  dif- 
ferentiation of  stem  and  leaves  resembles  to  a  re- 
markable degree,  externally,  the  corresponding 


36  Plant  Life  and  Evolution 

structures  in  the  highest  plants ;  but  it  is  perfectly 
clear  that  the  stem  and  leaves  in  the  seaweed  and 
the  seed-plant  have  no  genetic  connection,  but  have 
arisen  quite  independently  in  the  course  of  evolu- 
tion in  the  very  widely  separated  plants,  in  response 
to  the  same  needs.  We  meet  with  the  same  phe- 
nomenon again  among  the  mosses,  where  there  are 
developed  perfect  leaves  of  a  type  quite  different 
from  those  of  either  the  seaweed  or  the  flowering 
plant. 

The  extraordinary  variety  of  leaf  structures 
found  among  the  flowering  plants  can  usually  be 
correlated  with  special  adaptations  to  light  condi- 
tions, and  the  adaptability  of  individual  plants  in 
this  respect  is  extraordinarily  great.  A  further 
discussion  of  the  relations  of  plants  to  light  must 
be  left  for  another  chapter. 

Range  of  Temperature  Suitable  for  Plant  Growth. 
— An  indispensable  condition  for  the  manifestation 
of  life  in  any  organism  is  a  suitable  temperature. 
However,  there  may  be  a  great  deal  of  difference 
shown  by  different  plants  in  the  range  of  tempera- 
tures which  they  can  endure.  As  a  rule  all  mani- 
festations of  life  cease  in  plants  when  the  surround- 
ing medium  is  cooled  to  the  freezing  point  of  fresh 
water  (o°  C),  but  some  seaweeds  thrive  in  water 
which  may  fall  below  this  temperature  and  seldom 
has  its  temperature  much  above  o°  C.  Such  cold 
water  algae  are  quickly  killed  by  a  temperature 
only  a  few  degrees  above  the  freezing  point,  while 


Factors  in  Evolution  37 

other  low  forms  growing  in  hot  springs  like  those 
of  Yellowstone  Park  and  elsewhere  are  said  to  en- 
dure a  temperature  of  85°  C.  or  even  more.  Some 
bacteria,  also,  thrive  in  similar  high  temperatures 
which  would  be  immediately  fatal  to  most  plants. 
These  heat-loving  organisms  must  have  protoplasm 
of  somewhat  different  constitution  from  that  of 
most  plants,  as  a  temperature  much  lower  than  that 
at  which  they  thrive  causes  the  coagulation  of  the 
albuminous  contents  of  the  cells  of  most  plants,  and 
this  means  the  death  of  the  protoplasm. 

Among  the  higher  plants,  many  of  the  desert 
forms,  under  the  fierce  rays  of  an  unclouded 
tropical  sun,  must  be  exposed  to  temperatures 
which  would  quickly  destroy  the  protoplasm  of  the 
living  cells  were  these  not  amply  protected 
so  that  they  are  not  fully  exposed  to  the  un- 
tempered  heat  of  the  sun's  rays.  Naturally 
plants  of  warm  regions  differ  much  from  those 
of  colder  countries  in  the  range  of  temperature 
suitable  for  their  growth.  Thus  while  the  com- 
mon white  mustard  of  Northern  Europe  will  germ- 
inate at  a  temperature  near  the  freezing  point,  In- 
dian corn,  which  is  of  tropical  origin,  requires  9°  C. 
before  its  seeds  will  sprout.  The  optimum  tem- 
perature for  the  mustard  is  27°,  the  maximum  tem- 
perature which  it  will  endure  being  37°.  For  corn 
the  optimum  and  maximum  temperatures  are  re- 
spectively 34°  and  46°,  but  the  latter  figure  refers 
to  the  endurance  of  the  plant  when  placed  in  water 


38  Plant  Life  and  Evolution 

of  the  given  temperature.  Where  the  root  is  in 
the  soil  an  air  temperature  of  52°  can  be  borne. 

Modifications  Due  to  Temperature. — While  a 
suitable  temperature  is  necessary  for  normal  plant 
growth,  the  formative  effects  of  different  tem- 
peratures are  far  less  evident  than  those  due  to 
light.  Perhaps  the  most  evident  manifestations 
which  seem  to  be  due  to  temperature  are  the  pro- 
tective devices  seen  in  plants  of  cold  regions,  but 
these  same  effects  may  also  be  produced  by  defi- 
ciencies of  moisture.  The  deciduous  habit  and  the 
development  of  scale-clad  winter  buds  may  be  cited 
as  examples  of  such  temperature  modifications,  but 
very  similar  effects  may  be  noted  in  plants  living 
in  regions  where  there  is  a  marked  dry  season. 
Under  such  conditions  many  trees  shed  their  leaves 
in  much  the  same  way  that  they  do  in  colder  cli- 
mates. In  California,  for  example,  the  native  buck- 
eye casts  its  leaves  during  the  long  dry  summer,  and 
develops  resting  buds  quite  like  the  winter  buds  of 
the  forest  trees  of  Eastern  America. 

As  an  example  of  the  formative  effect  of  tem- 
perature among  the  lower  plants  can  be  cited  the 
experiments  of  Brefeld  upon  one  of  the  toad- 
stools (Coprinus).  In  this  fungus  it  was  found 
that  the  umbrella-shaped  fruiting  body  was 
formed  in  light  at  a  temperature  of  12°,  but  in 
darkness  its  development  required  15°.  So  also 
it  has  been  stated  that  certain  fern  spores,  which 
at  ordinary  temperatures  will  not  germinate  at  all  in 


Factors  in  Evolution  39 

darkness,  will  do  so  if  the  temperature  is  raised  to 
32°.  It  has  also  been  demonstrated  that  the  char- 
acter of  the  nutrition  may  affect  the  power  of  the 
plant  to  endure  higher  temperatures.  Thus  Theile 
found  that  the  common  blue  mold  (Penicillium) 
would  cease  to  grow  at  a  temperature  of  31°  when 
cultivated  in  a  sugar  solution,  but  would  endure 
a  temperature  of  from  35°  to  36°  when  fed  with 
formic  acid  and  glycerine. 

Endurance  of  Cold  by  Seeds. — There  seems  to 
be  no  degree  of  cold  which  is  sufficient  to  kill  dor- 
mant protoplasm.  Perfectly  dry  seeds  of  various 
kinds  have  been  exposed  to  the  temperature  of  liquid 
hydrogen  ( —  200°),  without  affecting  the  power  of 
germination  later  on  when  exposed  to  suitable  con- 
ditions. 

Relation  to  Water. — We  have  already  seen  that 
the  activity  of  protoplasm  is  dependent  upon  an  ade- 
quate water  supply,  and  therefore  the  presence  of 
water  is  essential  for  the  growth  of  all  plants.  The 
water  supply  is  perhaps  the  most  powerful  agency 
of  all  in  determining  the  form  of  the  plant  and  its 
organs.  In  its  normal  condition  the  cell  is  strongly 
distended,  and  tissues  composed  of  such  turgid  cells 
are  firm  and  elastic.  With  the  withdrawal  of  a 
portion  of  the  water  the  tissue  becomes  flaccid — • 
"  wilts,"  as  the  gardener  says  of  the  plant  which 
droops  for  lack  of  sufficient  water.  To  maintain 
the  tissues  in  their  normal  turgid  condition,  the  plant 
must  provide  for  a  loss  of  water  due  to  evapora- 


40  Plant  Life  and  Evolution 

tion,  and  for  this  purpose  there  have  been  developed 
in  the  higher  plants  special  organs,  roots,  for  the 
absorption  of  water.  There  have  also  arisen  ex- 
tremely efficient  tissues  for  its  distribution  through- 
out the  plant. 

Provision  has  also  to  be  made  for  checking  ex- 
cessive loss  of  water  through  evaporation.  Where 
plants  are  completely  submerged,  whether  they 
are  simple  seaweeds  or  more  highly  developed 
plants  like  the  pond-weeds,  eel  grass,  etc.,  there  is 
free  communication  between  the  water  within 
the  cells  and  that  outside  the  plant.  The  spe- 
cific gravity  of  the  plant  is  not  greatly  different 
from  that  of  the  surrounding  medium  which  buoys 
it  up  without  the  need  of  its  developing  special 
supporting  tissues,  and  the  seaweed  or  other  sub- 
mersed plant  collapses  completely  when  removed 
from  the  water  which  has  served  to  support  it. 

Modification  Due  to  Aquatic  Life. — The  differ- 
ence in  habit  of  the  same  species  grown  in  water 
and  out  of  it  is  strikingly  shown  in  a  good  many 
amphibious  plants.  Thus  the  common  yellow  pond 
lily  (Nuphar),  grown  in  deep  water,  has  a  leaf 
stalk  long  and  flexible,  and  the  leaves  lie  flat  upon 
the  surface  of  the  water.  The  same  plant  growing 
in  shallow  water,  or,  as  sometimes  occurs,  simply  on 
the  exposed  mud,  has  short  and  stout  leaf  stalks 
which  bear  the  narrower  and  firmer  leaves  com- 
pletely erect.  Another  common  aquatic  plant,  the 
arrow-head  (Sagittaria),  often  has  two  sorts  of 


Factors  in  Evolution  41 

leaves;  very  narrow  submersed  ones  with  no  blade, 
and  large  aerial  leaves  borne  on  stout  stalks  with 
broad  arrow-shaped  blade.  In  various  other 
aquatics,  like  the  water  crow-foot,  there  are  sub- 
mersed leaves  which  are  finely  divided,  while  the 
aerial  leaves  are  broad.  The  general  tendency  in 
submersed  leaves  is  to  assume  a  very  long  slender 
form,  or  to  be  divided  into  many  fine  segments. 
These  finely  divided  leaves  remind  one  of  the  fila- 
mentous structure  of  many  algae,  and  probably  this 
adaptation  is  efficient  in  exposing  a  larger  surface 
for  the  absorption  of  CO2  and  oxygen  in  the  water, 
where  these  substances  are  less  abundant  than  they 
are  in  the  atmosphere. 

So  long  as  the  plants  are  completely  submersed, 
loss  of  water  by  evaporation  is  impossible;  but  so 
soon  as  they  are  exposed  to  the  air  evaporation 
is  very  rapid  and  the  algse  or  other  aquatic 
plants  wither  very  quickly  when  thus  exposed. 
Where,  ho.wever,  algae  are  regularly  exposed  for  a 
certain  time,  as  happens  with  many  of  the  seaweeds 
that  grow  between  tide  marks,  they  usually  de- 
velop gelatinous  or  mucilaginous  substances  which 
retain  water  with  great  tenacity,  and  such  algae 
may  be  exposed  to  the  air  or  even  to  the  hot  sun 
for  several  hours  without  losing  sufficient  water  to 
injure  them.  When,  however,  a  plant  is  habitually 
exposed  to  the  air,  as  is  the  case  with  a  typical  land 
plant,  all  of  the  exposed  surfaces  develop  a  more  or 
less  thickened  cuticle  which  is  impervious  to  water. 


42  Plant  Life  and  Evolution 

Changes  Due  to  Terrestrial  Habit. — The  transla- 
tion to  land  involves  at  once  a  complete  readjust- 
ment of  the  plant  to  its  water  relation.  The  ex- 
posed surfaces  must  be  protected  against  excessive 
evaporation,  and  this,  of  course,  implies  a  diminu- 
tion of  the  power  of  absorbing  water  from  with- 
out. While  some  land  plants,  like  certain  Cali- 
fornian  mosses  and  ferns,  and  the  resurrection  plant 
(Selaginclla  lepidophylla)  of  Arizona,  can  absorb 
water  through  their  leaves,  thus  behaving  like  algae, 
this  is  not  possible  in  most  land  plants,  which  pro- 
cure their  water  mainly  through  the  root  system. 
The  need  for  rapid  distribution  of  water  through  the 
plant  is  met  by  the  development  of  an  elaborate 
"  fibre-vascular "  system  of  tissue  in  the  higher 
plants. 

The  ability  of  the  plant  to  regulate  the  loss 
of  water  through  evaporation  may  be  readily  dem- 
onstrated by  comparing  plants  of  the  same  species 
grown  under  different  conditions.  Where  moisture 
is  abundant  and  the  need  of  economy  small,  the 
rank-growing  plant  has  thick  stems  and  large 
thin  leaves  with  a  poorly  developed  cuticle.  This 
same  plant  grown  in  full  sunshine,  with  a  limited 
water  supply,  is  comparatively  small,  and  not  only 
is  there  a  smaller  surface  exposed  to  evaporation, 
but  the  surface  tissues  are  very  much  less  per- 
meable. These  latter  conditions  are  most  pro- 
nounced in  plants  of  hot  dry  regions  where  the 
problem  of  existence  is  concerned  first  of  all  with 


Factors  in  Evolution  43 

the  question  of  water.  It  is  not  always  easy  to  dis- 
tinguish between  modifications  due  to  light  and 
those  concerned  with  the  water  supply.  Thin,  broad 
shade  leaves  are  correlated  with  the  necessity  of  a 
greater  amount  of  chlorophyll  tissue,  owing  to  the 
less  powerful  illumination,  and  also  with  the  les- 
sened evaporation  due  to  the  moisture  in  the  air  and 
lower  temperature  of  the  deep  shade.  Many  of  the 
lower  plants,  such  as  the  algae  and  mosses  which 
are  not  able  to  survive  complete  drying  up,  but 
which  must  provide  against  any  such  contingency, 
have  evolved  reproductive  cells — spores — which  are 
fitted  to  resist  prolonged  desiccation.  In  the  seed- 
plants  there  have  also  been  developed  special  struc- 
tures— seeds,  bulbs,  tubers,  etc. — which  enable  these 
plants  to  pass  unharmed  through  periods  of  drought 
or  cold  which  are  fatal  to  the  plant  in  its  active 
condition.  These  reproductive  structures  absorb 
water  promptly  and  begin  to  grow  when  proper  con- 
ditions are  presented. 

Formative  Effects  of  Gravity. — The  continually 
acting  force  of  gravity  undoubtedly  exercises  a 
powerful  formative  effect  upon  all  plant  structures, 
but  the  nature  of  this  influence  is  still  very  obscure. 
As  a  rule  aerial  shoots  grow  upwards,  and  are  said 
to  be  negatively  geotropic,  while  roots  growing 
downward  are  positively  geotropic.  A  young  seed- 
ling placed  horizontally  will  quickly  show  an  up- 
ward curvature  of  the  shoot  and  a  downward  bend- 
ing of  the  root.  Should  the  force  of  gravity  be 


44  Plant  Life  and  Evolution 

eliminated  by  revolving  the  plant  so  that  all  por- 
tions are  equally  exposed  to  the  influence  of  gravity 
this  curvature  will  be  absent.  The  many  mechanical 
stresses  due  to  the  action  of  gravity  must  exercise 
a  pronounced  influence  upon  the  growth  of  all  or- 
gans exposed  to  them.  For  example,  the  continual 
pull  due  to  the  weight  of  a  leaf  or  branch  must 
react  upon  the  growing  cells  of  the  organs  and  in- 
fluence the  development  of  tissues. 

The  Necessary  Chemical  Constituents  of  Plants. 
— The  growth  of  plants  is  dependent  on  the  pres- 
ence of  certain  chemical  elements  which  are  indis- 
pensable as  food,  or  are  in  some  way  bound  up  with 
the  constructive  processes  going  on  in  the  plant. 
The  growth  of  the  plant  is  often  very  greatly  af- 
fected by  the  presence  or  absence  of  certain  elements 
which  form  a  very  insignificant  part  of  its  sub- 
stance, but  which  are  evidently  indispensable  in  the 
processes  of  growth.  Thus  a  plant  cultivated  in  an 
artificially  made  nutrient  solution  containing  all  the 
essential  chemical  elements  except  potassium  or  iron, 
will  show  very  little  growth,  although  the  amount 
of  iron  or  potassium  present  in  the  tissues  is  ex- 
cessively small;  but  if  entirely  deprived  of  these  ele- 
ments the  plant  is  quickly  dwarfed  and  growth  will 
be  almost  entirely  suspended. 

The  movements  of  motile  plant  cells  may  be  mark- 
edly affected  by  the  attractive  power  of  certain  sub- 
stances, and  this  chemical  attraction  has  been  named 
Chemotaxis.  Thus  many  bacteria  will  quickly 


Factors  in  Evolution  45 

move  towards  the  source  of  oxygen,  and  under  a 
cover  glass  may  be  seen  to  assemble  near  the 
edge  of  the  glass  where  the  oxygen  is  more  abun- 
dant. They  also  will  quickly  gather  about  green 
plant  cells  where  oxygen  is  being  given  off  under 
the  influence  of  light.  Bacteria  are  also  power- 
fully attracted  by  certain  organic  solutions.  Chemo- 
taxis  plays  an  important  part  also  in  the  attraction 
of  the  motile  male  cells,  or  sperms,  to  the  eggs.  It 
has  been  shown  by  Pfeffer,  in  the  case  of  some  of 
the  ferns,  that  when  the  archegonium,  the  organ 
containing  the  egg  cell,  opens,  a  substance  is  ejected 
which  contains  some  combination  of  malic  acid,  and 
this  exercises  a  powerful  attraction  upon  the  sperms 
in  response  to  which  they  quickly  crowd  into  the 
open  archegonium  and  thus  reach  the  egg  within 
it.  A  suitable  solution  of  malic  acid  placed  in  a 
fine  glass  tube  will  cause  the  free  sperms  to  swim 
into  it  much  as  they  do  into  the  open  archegonium. 
The  investigations  of  Klebs  have  shown  that  in 
many  of  the  lower  plants  the  character  of  the  re- 
productive cells  may  be  largely  controlled  by  the 
nature  of  the  medium  in  which  the  plants  are  grown. 
Thus  in  a  low  alga,  the  water  net  (Hydro- 
dictyon),  the  formation  of  the  non-sexual  repro- 
ductive cells,  zoospores,  may  be  induced  by  culti- 
vating the  plants  in  a  solution  of  maltose,  while  if 
placed  in  a  solution  of  cane-sugar,  there  is  an  ex- 
traordinary development  of  the  sexual  reproductive 
cells.  In  this  connection  we  may  also  cite  the  in- 


46  Plant  Life  and  Evolution 

vestigations  of  Loeb  in  artificial  parthenogenesis, 
in  which  he  has  shown  that  certain  chemical  stimuli 
may  replace  to  a  certain  extent  the  function  of  the 
spermatozoa  in  normal  fertilization. 

SELECTION 

The  last,  but  not  the  least,  factor  to  consider  is 
selection.  After  a  variation  has  appeared,  whatever 
may  have  been  its  cause,  it  remains  to  be  seen 
whether  or  not  such  a  variation  has  sufficient  po- 
tency to  maintain  itself.  The  deciding  factor  in 
the  persistence  and  cumulation  of  any  line  of  varia- 
tion is  Natural  Selection,  which  is  the  final  arbiter 
in  determining  what  forms  shall  survive  as  species 
or  groups  of  species. 


CHAPTER  III 
THE  LOWER  PLANTS 

IN  endeavoring  to  trace  the  pedigree  of  the  vege- 
table kingdom,  we  can  rely  on  the  record  of 
the  past  only  to  a  limited  extent.  While  many 
plants  have  left  perfectly  recognizable  fossil  re- 
mains, the  record  is  extremely  fragmentary.  This  is 
especially  true  of  the  delicate  and  more  perishable 
plants,  such  as  the  seaweeds  and  mosses.  Neverthe- 
less most  important  results  have  been  obtained  from 
the  careful  study  of  fossil  plant  remains. 

Comparative  Morphology  as  a  Guide  to  Relation- 
ships.— On  the  assumption  that  all  plants  are  more 
or  less  closely  related,  a  comparison  of  the  structures 
of  the  living  forms  affords  a  clue  to  the  degree  of 
relationship,  and  hence  the  great  stress  which  is 
laid  upon  the  importance  of  Comparative  Morphol- 
ogy. In  view,  however,  of  the  ready  response  of 
plant  structures  to  changes  in  environment,  great 
caution  must  be  exercised  in  distinguishing  true 
homologies  from  similarities  in  structure  due  to  re- 
sponse to  similar  conditions.  We  have  already 
pointed  out,  for  instance,  that  leaf-like  organs  have 
developed  in  plants  of  widely  separate  origin,  e.g., 
seaweeds,  mosses,  and  the  higher  land  plants.  These 

47 


48  Plant  Life  and  Evolution 

leaves  are  in  no  sense  homologous  organs  and  do  not 
point  to  any  close  relationship  between  the  plants 
which  possess  them.  A  proper  study  of  comparative 
morphology  must  take  into  account  all  the  organs  of 
the  forms  compared,  but  it  also  is  evident  that  some 
of  these  organs  are  much  more  important  in  hered- 
ity than  others.  It  is  also  necessary  to  distinguish 
between  structures  which  are  readily  affected  by  ex- 
ternal conditions  and  those  which  show  evidence  of 
being  more  permanent  in  character.  The  repro- 
ductive parts  are  as  a  rule  much  more  stable  than 
the  vegetative  organs,  and  are  rightly  considered  to 
hold  the  first  place  in  indicating  affinities  between 
plants.  For  example,  the  vegetative  organs,  espe- 
cially the  leaves,  are  rudimentary  in  such  a  plant  as 
the  Indian  pipe  (Monotropa),  which  has  entirely  lost 
the  power  of  the  photosynthesis  and  feeds  on  organic 
substances.  The  flower,  however,  is  but  little  altered, 
and  shows  its  unmistakable  relationship  to  the  rho- 
dodendron, huckleberry,  and  other  heath-like  plants. 
Embryology. — The  study  of  the  development  of 
the  plant  and  of  its  different  organs,  or  embryology, 
in  its  wider  sense,  is  of  great  importance.  Here 
also  great  care  must  be  taken  in  determining  what 
are  primitive  characters  and  what  are  merely  adapta- 
tions. Of  late,  much  attention  has  been  given  to 
Experimental  Morphology,  or  the  critical  study  of 
the  direct  effects  of  various  stimuli  upon  the  de- 
veloping organism.  While  there  is  good  reason  to 
suppose  that  much  valuable  information  as  to  the 


The  Lower  Plants  49 

factors  governing  the  development  of  plants  is  to 
be  obtained  from  such  experimental  work,  the  prob- 
lems are  so  exceedingly  complicated  that  great  cau- 
tion must  be  exercised  in  drawing  general  con- 
clusions. Our  ignorance  of  so  many  of  the  condi- 
tions that  have  governed  the  evolution  of  any  plant 
in  the  past,  and  the  long  time  that  may  elapse  before 
the  reactions  resulting  from  a  stimulus  show  them- 
selves, emphasize  the  necessity  for  extreme  cau- 
tion in  making  sweeping  generalizations  from  the 
results  of  any  experiments.  Nevertheless  such  work 
is  exceedingly  valuable  in  checking  and  extending 
the  results  derived  from  other  sources,  such  as 
paleontology  and  comparative  morphology. 

There  still  exist  organisms  that  may  very  well 
be  not  very  different  from  the  first  living  things 
that  appeared  upon  the  earth.  These  are  the 
Bacteria,  those  ubiquitous  "  germs,"  cells  so  minute 
that  under  the  most  powerful  microscope  many  of 
them  appear  merely  as  tiny  specks  too  small  to  show 
any  definite  structure. 

These  minute  organisms,  however,  are  of  the 
highest  importance  in  the  economy  of  nature,  as  it 
is  to  their  activity  that  most  forms  of  organic  de- 
composition are  due,  and  this  decomposition  is  es- 
sential in  order  that  inorganic  compounds  may  be 
reduced  to  simpler  ones  that  are  available  as  food 
for  the  higher  plants.  In  the  earth  there  are  myri- 
ads of  other  bacteria  whose  activity  results  in  the 
fixing  of  the  atmospheric  nitrogen,  and  the  produc- 


50  Plant  Life  and  Evolution 

tion  of  nitrogenous  compounds  suitable  for  plant 
food. 

Blue-green  Algae. — Probably  related  to  some  of 
the  bacteria  are  certain  common  plants  of  low  or- 
ganization, called,  from  their  color,  the  blue-green 
algse  (Cyanophyceae),  and  these,  with  the  bacteria, 
constitute  a  group  of  plants  known  as  the  Schi- 
zophyta,  or  fission  plants,  as  their  only  form  of 
propagation  is  by  means  of  simple  cell  division. 
Some  of  these  blue-green  algse  occur  as  slimy,  black- 
ish green  films  on  wet  earth  or  on  objects  in  the 
water,  while  others  give  rise  to  jelly-like  masses, 
often  of  considerable  size,  in  which  the  plants  are 
imbedded.  Some  of  these  organisms  occur  in  hot 
springs  like  those  in  the  Yellowstone  Park,  and  like 
many  of  the  bacteria  they  are  able  to  endure  tem- 
peratures which  are  fatal  to  most  plants.  Like  the 
bacteria,  also,  the  blue-green  algse  show  a  very 
primitive  cell  structure,  and  are  presumably  very 
ancient  types,  but  it  is  by  no  means  certain  that 
either  these  forms  or  the  bacteria  are  related  directly 
to  the  higher  plants. 

Flagellata. — Many  bacteria  are  actively  motile, 
being  provided  with  cilia  so  that  they  resemble  some- 
what the  low  organisms  known  as  the  Flagellata. 
The  Flagellata  are  also  presumably  very  primitive 
organisms.  In  the  structure  of  their  cells  they  re- 
semble the  lower  types  of  plants  and  animals,  and 
there  is  some  reason  to  suppose  that  they  more 
closely  resemble  the  progenitors  of  both  the  higher 


The  Lower  Plants  51 

plants  and  animals  than  do  any  other  existing  or- 
ganisms. The  flagellates  were  long  looked  upon  as 
a  division  of  the  Infusoria,  the  most  important 
group  of  unicellular  animals,  but  recent  studies  on 
these  forms  have  shown  that  there  are  two  well- 
marked  types,  one  of  which  possesses  the  chromato- 
phores  or  chlorophyll-corpuscles  characteristic  of 


FIG.  3 

A  colorless  flagellate,  Bodo  caudatus,  attacking  a  flagellate 
unicellular  plant,  Chlamydomonas,  which  possesses  a  green 
chromatophore,  cr.  A  nucleus,  n,  is  present  in  each  organism. 
(After  Butschli.) 

the  typical  plants,  while  in  the  others  the  chromato- 
phores  are  wanting.  All  flagellates  possess  cilia 
or  flagella,  by  means  of  which  they  are  propelled 
through  the  water.  The  green  flagellates  closely 
resemble  many  of  the  lower  green  plants,  which 
often  give  rise  to  free  swimming  reproductive  cells 
very  closely  resembling  these  flagellates  and  sug- 
gesting that  the  latter  are  related  to  the  ancestors 
of  the  higher  green  algae.  On  the  other  hand,  the 


52  Plant  Life  and  Evolution 

colorless  flagellates  show  a  close  structural  resem- 
blance to  the  cells  of  such  low  animal  types  as  the 
simple  sponges,  and  the  derivation  of  these  animals 
from  flagellate  ancestors  is  readily  conceivable.  The 
flagellates  may  thus  be  said  to  represent  an  ex- 
tremely ancient  structural  type  from  which  have 
been  developed,  on  the  one  hand  colorless  animal 
forms,  and  on  the  other  green  plants. 

The  pigment-bearing  flagellates  show  a  tendency 
to  approach  the  non-motile  condition  of  the  typical 
plants.  This  tendency  toward  the  immobile  condi- 
tion may  perhaps  be  correlated  with  the  presence  of 
chlorophyll  and  the  resulting  ability  to  assimilate 
inorganic  food.  The  colorless  flagellates,  not  hav- 
ing this  property,  would  require  greater  power  of 
movement  in  their  search  for  food;  i.e.,  their  nutri- 
tion is  typically  animal.  With  the  establishment  of 
these  two  great  lines  of  development — plants  and 
animals — the  characteristics  of  each  become  more 
pronounced,  and  above  the  flagellates  any  organ- 
ism may  be  easily  referred  either  to  the  animal  or 
vegetable  kingdom. 

Volvocales. — In  studying  the  various  forms  of 
life  that  abound  in  fresh- water  ponds,  we  often 
find,  actively  swimming  about,  certain  green  cells 
which  may  be  solitary,  or  which  may  be  united 
in  globular  bodies  large  enough  to  be  seen  with  the 
naked  eye.  The  best  known  of  these  organisms  is 
Volvox,  whose  globular  body  may  contain  thou- 
sands of  cells  and  is  easily  seen.  Most  of  these 


The  Lower  Plants  53 

"  Volvocales "  are  composed  of  a  much  smaller 
number  of  cells,  and  some  of  them  are  unicellular 
(see  Fig.  3).  Under  certain  conditions,  the  cell 
divisions  take  place  with  great  rapidity  and  within 
a  few  days  a  small  pool  or  watering  trough  may 
have  its  waters  made  quite  green  by  the  presence 
of  millions  of  these  little  plants. 

The  structure  of  the  cells  in  Volvox  is  like  that  of 
most  of  the  lower  green  plants.  Each  cell  has  a 
single  large  green  chromatophore.  There  is  a  dis- 
tinct nucleus  and  there  is  also  a  peculiar  body,  red 
in  color,  and  apparently  an  organ  for  light  percep- 
tion, hence  often  known  as  the  eye  spot.  These 
organisms  are  extremely  sensitive  to  light,  and  by 
means  of  the  two  cilia  with  which  each  cell  is 
provided  they  swim  toward  the  source  of  light. 
This  moving  toward  the  light  is  undoubtedly  asso- 
ciated with  the  function  of  photosynthesis. 

New  plants  arise  by  division  of  special  cells,  and 
as  we  have  seen,  these  divisions  may  follow  very 
rapidly,  so  that  the  number  of  individuals  may  in- 
crease enormously  in  a  very  short  time.  Another 
form  of  reproduction,  however,  occurs  at  the  end 
of  the  growing  season,  at  which  time  there  are  de- 
veloped special  structures  fitted  to  survive  the  drying 
up  of  the  water,  which  is  fatal  to  the  plant  in  its 
active  condition.  These  structures  are  the  "  rest- 
ing spores,"  which  are  provided  with  a  thick  cell 
wall  and  contain  an  accumulation  of  food  materials, 
so  that  they  are  thus  protected  against  the  effects 


54  Plant  Life  and  Evolution 

of  drought,  and  the  food  supply  within  the  cells 
permits  of  very  rapid  germination  when  the  proper 
time  comes.  These  "  resting  spores  "  are  the  result 
of  a  simple  sexual  process.  Two  special  cells,  the 
"  gametes  "  or  sexual  cells,  fuse  into  a  single  one, 
and  the  resulting  cell,  or  "  zygote,"  becomes  the  rest- 
ing spore.  In  the  unicellular  forms,  the  gametes  are 
alike,  both  being  provided  with  cilia  and  hardly  dis- 
tinguishable from  the  ordinary  individuals;  but  in 
the  higher  forms,  like  Volvox,  one  of  the  gametes 
is  a  very  small  and  active  cell,  the  male  or  sperm 
cell,  while  the  other  is  very  much  larger  and  is  desti- 
tute of  cilia.  The  latter  is  the  female  cell — egg,  or 
ovum — and  when  fertilized  by  the  sperm,  develops 
into  the  resting  spore. 

Because  of  their  permanently  motile  condition, 
and  their  unmistakable  resemblance  to  the  Flagel- 
lata,  the  Volvocales  are  sometimes  claimed  by  zo- 
ologists as  animals;  but  the  cell  structure  and  the 
reproduction  are  in  all  respects  like  those  of  the 
lower  algae,  and  there  is  no  question  but  that  the 
relationship  with  the  latter  is  a  very  close  one. 

Among  the  unicellular  Volvocales  it  is  not  un- 
common to  find  the  plants  assuming  a  quiescent  con- 
dition. The  cilia  are  lost,  and  the  cell  then  closely 
resembles  many  of  the  ordinary  unicellular  algae. 
It  is  highly  probable  that  the  lower  green  algae 
originated  in  some  such  fashion  from  unicellular 
Volvocales,  this  being  indicated  by  the  frequent 
reversion  in  so  many  algae  to  the  motile  condition 


The  Lower  Plants  55 

in  the  form  of  zoospores,  which  are  identical  in 
structure  with  the  cells  of  Volvox. 

Multicellular  Algae. — While  a  good  many  of  the 
lower  algae  remain  unicellular,  more  of  them  are 
multicellular  organisms.  Among  the  simplest  of 
these  multicellular  algae  are  the  various  "  pond- 
scums,"  those  soft,  foamy  green  masses  so 
often  seen  floating  on  stagnant  water.  These 
plants  consist  of  chains  of  entirely  similar  cells 
floating  free  in  the  water.  Others  of  these 
low  algae,  such  as  the  one  shown  in  Fig.  4,  B,  are 
attached,  and  the  basal  cell  may  develop  root- 
like  outgrowths  so  as  to  form  a  definite  organ  of 
attachment.  These  plants  often  multiply  by  means 
of  zoospores,  little,  naked,  free-swimming  cells 
closely  resembling  the  cells  of  Volvox.  After  a 
short  period  of  activity,  these  zoospores  settle  down, 
develop  a  cell  wall,  and  thus  for  a  short  period  as- 
sume a  typically  unicellular  condition.  By  repeated 
divisions,  this  cell  then  gives  rise  to  the  character- 
istic filament,  or  cell  row.  It  will  be  seen,  then,  that 
in  its  development  the  plant  passes  successively 
through  the  free,  motile  stage,  and  the  stationary 
unicellular  condition,  before  it  finally  attains  its 
adult  multicellular  state. 

Photosynthetic  Organs  in  Algae. — From  these 
simple  beginnings  there  may  be  traced  many  inter- 
mediate stages  leading  up  to  the  branched  or  broad 
flattened  bodies  which  distinguish  the  more  compli- 
cated types  of  algae.  This  increase  in  the  complex- 


56  Plant  Life  and  Evolution 

ity  of  the  plant  body  is  usually  associated  with  the 
increase  in  the  amount  of  green  tissue.  The  most 
important  function  of  the  plant's  life,  aside  from 


FIG.  4 

Evolution  of  the  plant  body  in  the  lower  green  plants. 

A — Unicellular  plant  (Pleurococcus). 

B — Unbranched  filament,  with  basal  root-like  organ,  or  hold 
fast  (r)  (CEdogonium). 

C— A  confervoid  alga,  Draparnaldia,  in  which  there  is  a 
photosynthetic  apparatus  consisting  of  tufts  of  densely 
crowded  green  cells,  Cl. 

reproduction,  is  the  process  of  photosynthesis ;  hence 
the  adjustment  of  the  green  cells  to  the  light  ex- 
posure is  of  prime  importance,  and  this  adjustment 
explains  some  of  the  most  striking  modifications  of 


The  Lower  Plants  57 

the  primitive  plant  body.  In  such  forms  as  that 
shown  in  Fig.  4,  C,  the  chlorophyll  is  mainly  re- 
stricted to  small  cells,  whose  number  is  increased 
by  the  extensive  branching  of  the  filaments,  so  that 
there  arise  dense  tufts  of  small  green  cells,  which 
might  very  well  be  compared  to  a  finely  divided  leaf, 
and  like  the  leaf,  these  may  be  fairly  described  as 
special  photosynthetic  organs.  The  flat,  leaf-like 
body  of  the  common  sea-lettuce  (Ulva)  and  many 
other  algae  illustrate  adaptation  by  which  the  amount 
of  green  tissue  exposed  to  the  light  is  increased. 

Green  Algae;  Chlorophyceae. — The  primitive 
fresh- water  algae  are  none  of  them  of  very  great 
size,  and  for  the  most  part  they  retain  a  simple 
structure.  Their  color  is  usually  a  vivid  green,  un- 
obscured  by  the  red  and  brown  pigments  which  dis- 
tinguish most  of  the  large  seaweeds.  These  pure 
green  forms  are  therefore  known  as  the  "  Green 
Algae,"  or  Chlorophyceae.  While  most  of  these  are 
confined  to  fresh  water,  some  of  them  have  migrated 
to  the  sea,  and  in  the  altered  environment  have 
undergone  more  or  less  marked  changes.  The  most 
striking  of  these  green  seaweeds  are  those  known  as 
the  Siphoneae,  an  assemblage  of  very  peculiar  green 
plants  distinguished  by  an  almost  complete  suppres- 
sion of  cell  division,  so  that  the  plants  are  composed 
of  a  system  of  open,  often  very  much  twisted  and 
branched  tubes,  without  any  division  walls.  A  few 
of  these  curious  plants  occur  in  the  cooler  seas,  but 
to  observe  them  in  their  fullest  development  one 


58  Plant  Life  and  Evolution 

must  visit  the  warm  seas  of  the  Tropics,  where, 
especially  about  the  coral  reefs,  these  interesting 
plants  abound.  Some  of  them  are  fan-shaped,  flat- 
tened bodies;  others  jointed,  much  branched,  coral- 
like  plants,  sometimes  having  the  ends  of  the 
branches  tipped  with  bright  green  tufts  of  hairs 
looking  like  the  tentacles  of  some  polyp.  Still  others 
have  creeping  stems  from  which  arise  fern-like 
leaves  and  send  down  into  the  coral  sand  fine, 
branching  roots.  These  plants  might  almost  be 
called  plant-corals,  since,  like  the  true  corals,  they 
possess  a  calcareous  skeleton  and  play  a  more  or 
less  important  part  in  building  up  the  coral  reefs 
where  they  grow.  Like  the  animal  corals,  also,  they 
have  been  found  abundantly  in  a  fossil  condition, 
and  there  is  some  evidence  that  they  already  existed 
in  the  ancient  Silurian  seas.  For  a  long  time  these 
remains  were  supposed  to  be  those  of  animals,  and 
it  is  only  of  late  that  the  real  nature  of  these  fossil 
Siphoneae  has  been  recognized. 

Green  Algae  Mostly  Fresh-water  Types. — With 
the  exception  of  the  Siphoneae,  most  of  which 
are  marine  plants,  the  majority  of  the  green  algae 
are  fresh-water  types.  There  are,  however,  two 
great  divisions  or  classes  of  algae  which  constitute 
the  bulk  of  the  marine  vegetation,  at  least  along  the 
shore.  These  are  the  Brown  Algae,  or  Phaeophyceae, 
and  the  Red  Algae,  or  Rhodophyceae,  both  of  which 
reach  greater  dimensions  than  any  of  the  green 
algae,  and  are  much  more  specialized.  These  are 


The  Lower  Plants  59 

the  seaweeds  par  excellence.  Of  these  two  groups 
the  Phaeophyceae  are  almost  exclusively  marine, 
while  the  red  algae  have  a  number  of  fresh- water 
representatives. 

The  Brown  Algae. — Very  different  from  the  small 
and  delicate  green  algae  are  the  great  coarse  brown 
kelps  so  common  along  the  rocky  coasts  of  the  cooler 
seas.  The  rocks  of  the  northern  New  England  shore 
exposed  by  the  tide,  are  covered  with  a  thick  drapery 
of  the  common  rock-weed,  or  bladder  kelp,  and  in 
the  deeper  water  are  groves  of  the  big  Laminarias, 
"  devil's  aprons  "  in  the  vernacular.  With  these  are 
many  species,  some  delicate  little  plants,  but  more 
of  them  stout  and  leathery  in  texture,  and  all  dis- 
tinguished from  the  green  algae  by  the  presence  of 
brown  or  yellow  pigments  which  give  them  their 
characteristic  olive  or  leathery  brown  color.  Much 
the  same  types  occur  on  the  western  European 
coasts,  but  it  is  in  the  Pacific  that  these  remarkable 
plants  reach  their  greatest  development.  The  visitor 
to  the  Pacific  Coast,  from  Alaska  to  Middle  Cali- 
fornia, is  at  once  struck  by  the  extraordinary  variety 
and  gigantic  size  of  some  of  the  common  kelps, 
which  attain  a  length  hardly  rivaled  by  any  land 
plants,  and  make  these  great  seaweeds  the  giants 
of  their  class. 

Giant  Kelps. — Compared  with  these  giants  of  the 
Pacific,  the  largest  of  the  Atlantic  kelps  are  mere 
pigmies.  Two  of  the  Pacific  kelps,  Nereocystis  and 
Macrocystis,  especially  merit  the  popular  name  of 


60  Plant  Life  and  Evolution 

Giant  Kelps.  The  former  is  a  most  striking  sea- 
weed of  the  Northern  Pacific  Coast,  where  it  is  said 
to  reach  a  length  of  one  hundred  meters.  Its  slen- 
der stem  is  firmly  anchored  in  deep  water,  reaching 
to  the  surface,  where  it  expands  a  cluster  of  broad 
leaves  two  or  three  meters  in  length,  and  buoyed 
up  by  a  great  bladder-like  float  as  big  as  a  croquet 
ball.  Macrocystis  is  said  to  reach  even  a  greater 
length  than  Nereocystis,  and  great  beds  of  this  kelp 
occur  off  the  California  Coast.  These  beds  of  kelp, 
as  for  instance  at  Santa  Barbara,  form  very  efficient 
breakwaters. 

Some  of  the  kelps,  like  the  curious  sea-palm 
(Postelsia)  (Fig.  20),  are  specially  adapted  to 
growth  in  the  heaviest  surf,  and  seek  the  most  ex- 
posed rocks,  where  they  are  subjected  to  the  full 
force  of  the  great  Pacific  rollers. 

Gulf  Weed. — Other  striking  types  of  the  brown 
algae  are  the  floating  forms  like  the  gulf-weed 
(Sargassum)  of  the  Caribbean  Sea,  which  is  drifted 
northward  by  the  Gulf  Stream  and  is  familiar 
enough  to  transatlantic  voyagers.  Many  similar 
forms  occur  in  the  warmer  seas,  and  they  are  espe- 
cially abundant  off  the  coast  of  Japan.  It  is  still 
not  settled  whether  all  of  these  floating  species  begin 
life  as  attached  plants,  and  are  subsequently  torn 
from  their  moorings.  That  they  live  for  a  very 
long  time  in  the  floating  condition  is  shown 
by  the  long  distances  from  land  at  which  they 
occur,  in  a  vigorously  growing  condition.  Some- 


The  Lower  Plants  61 

times  these  little  floating  islands  are  veritable  zo- 
ological gardens,  as  a  great  variety  of  small  marine 
animals,  sometimes  including  fish,  seek  shelter 
among  the  fronds  of  the  kelp. 

The  brown  algse  are  preeminently  marine  plants. 
Very  rarely  do  they  invade  fresh  water,  and  then 
only  in  the  immediate  vicinity  of  the  sea.  A  large 
number  of  them  are  plants  living  between  tide 
marks,  and  they  show  very  perfect  adaptation  to 
this  condition.  Their  leathery,  gelatinous  fronds 
retain  the  water  very  tenaciously,  and  they  may  be 
exposed  to  the  air  for  a  long  time  without  injury. 
Indeed  in  the  far  northern  regions  like  Alaska, 
where  as  a  rule  the  air  is  cool  and  moist,  they  often 
grow  so  near  the  high  tide  mark  as  to  be  out  of  the 
water  for  the  greater  part  of  the  time.  It  is  sup- 
posed that  the  brown  pigment  with  which  they  are 
provided  is  a  protection  against  too  great  illumina- 
tion when  they  are  uncovered  by  the  receding  tide. 

These  large  brown  algse  sometimes  show  a 
marked  degree  of  specialization.  The  massive, 
tough,  and  leathery  plants  often  develop  structures 
resembling  the  stem,  root,  and  leaves  of  the  higher 
plants.  As  they  very  often  grow  where  they  are 
exposed  to  the  full  force  of  the  ocean  surf,  they  are 
provided  with  powerful  root-like  organs  or  hold- 
fasts, by  which  they  cling  tenaciously  to  the  rocks. 
Thin  flat  leaf-like  organs  are  of  common  occurrence, 
and  in  connection  with  these  there  may  be  floats, 
or  air  vesicles,  which  buoy  up  the  leaves  and  keep 


62  Plant  Life  and  Evolution 

them  near  the  surface  of  the  water,  where  they  may 
have  proper  exposure  to  the  light.  In  these  great 
plants  there  is  also  internal  differentiation  shown, 
especially  in  the  occurrence  of  elongated  elements, 
which  recall  the  conducting  tissues  of  the  higher 
plants,  and  presumably  serve  the  same  purpose. 

There  is  no  reason  to  suppose  that  the  similarities, 
either  of  external  form  or  internal  structure,  point 
to  any  near  relationship  between  the  brown  sea- 
weeds and  any  land  plants;  These  resemblances, 
however,  illustrate  in  a  most  striking  way  the  forma- 
tion, in  two  widely  separate  groups  of  organisms, 
of  very  similar  structures  in  response  to  similar 
needs.  It  is,  in  short,  a  case  of  "  analogy  "  com- 
parable to  the  formation  of  functionally  similar, 
and  structurally  different,  organs  in  animals;  as, 
for  instance,  the  wings  of  birds  and  insects. 

Reproduction  of  Brown  Algae. — Some  of  the 
brown  algse  are  purely  asexual  in  their  reproduc- 
tion, while  others,  like  the  rock-weed  (Fucus),  have 
perfectly  developed  sexual  cells.  A  remarkable  fact 
is  that  the  forms  which  are  the  largest,  and  struc- 
turally the  most  complete,  i.e.,  the  giant  kelps,  are, 
so  far  as  is  known,  propagated  mainly  by  non- 
sexual  zoospores. 

Brown  Algae  Probably  Not  Related  to  the  Green 
Algae. — The  origin  of  the  brown  algae  is  by  no 
means  certain.  At  present  the  available  evidence 
seems  to  indicate  that  they  are  not  directly  related  to 
any  of  the  green  algae,  but  constitute  a  quite  inde- 


The  Lower  Plants  63 

pendent  developmental  line,  derived  from  some  free- 
swimming  type,  perhaps  allied  to  the  Flagellata. 
The  zoospores  or  asexual  reproductive  cells  of  these 
plants  differ  from  those  of  the  green  algae  in  hav- 
ing cilia  laterally  inserted  and,  of  course,  possessing 
a  brown  pigment.  Among  the  lower  alga-like 
forms  are  certain  types  known  as  Peridineae,  the 
simpler  forms  of  which  are  not  unlike  the  zoospores 
of  the  brown  algae,  having  like  them  two  laterally 
placed  cilia,  and  it  is  possible  that  the  beginning  of 
the  line  which  culminates  in  the  great  kelps  and 
rock-weeds  is  to  be  found  in  forms  resembling  the 
simpler  Peridineae,  which  in  turn  are  presumably 
allied  to  the  flagellates. 

Another  group  which  is  sometimes  associated 
with  the  flagellates  is  that  of  the  Diatoms,  which 
offer  a  large  and  widely  distributed  assemblage 
of  unicellular  plants,  occurring  everywhere,  both 
in  fresh  and  salt  water.  These  diatoms,  together 
with  the  Peridineae,  are  the  most  important  con- 
stituents of  the  floating  vegetation  of  the 
sea,  or  the  "  plankton  "  upon  which  the  animal  life 
of  the  ocean  very  largely  depends.  If  any  relation- 
ship really  exists  between  the  diatoms  and  the 
higher  Phaeophyceae  it  must  be  extremely  remote,  the 
diatoms  themselves  giving  some  evidence  that  they 
are  a  highly  specialized  group  of  comparatively  re- 
cent origin. 

The  Red  Algae. — The  very  characteristic  red 
algae,  while  they  comprise  a  majority  of  the  sea- 


64  Plant  Life  and  Evolution 

weeds,  are  far  less  conspicuous  than  the  great  kelps, 
owing  both  to  their  much  smaller  size,  and  to  their 
growing,  as  a  rule,  in  deeper  water,  or  under  the 
shelter  of  larger  seaweeds  and  rocks,  where  they  are 
easily  overlooked.  They  include  some  of  the  most 
exquisite  of  all  plants,  and  their  beautiful  tints  and 
graceful  forms  are  familiar  to  every  one  who  has 
made  even  a  casual  study  of  marine  plants. 

The  rose-red  pigment  which  quite  hides  the  green 
chlorophyll  in  the  living  plant  is  easily  extracted  by 
fresh  water,  and  then  the  presence  of  chlorophyll 
is  plainly  seen.  This  red  pigment  probably  supple- 
ments the  chlorophyll  in  the  process  of  photosynthe- 
sis, and  enables  the  chlorophyll  bodies  to  absorb  cer- 
tain light  rays  which  would  otherwise  be  unavail- 
able owing  to  the  deep  water  in  which  they  grow. 

A  marked  peculiarity  of  the  red  seaweeds  is  the 
complete  absence  of  any  motile  reproductive  cells, 
such  as  are  so  common  in  the  green  and  brown 
algae.  The  result  of  fertilization  is  not  a  single 
spore  which  directly  or  indirectly  produces  a  new 
plant,  but  there  is  formed  a  multicellular  structure, 
or  "  spore  fruit,"  which  by  budding  gives  rise  to 
many  spores.  The  complete  absence  of  any  motile 
cells  in  the  red  algae  is  difficult  to  explain,  as  it  is 
hard  to  see  what  advantage  this  can  be  to  the  plant. 

The  red  algae  are  not  so  exclusively  marine  as 
the  brown  seaweeds,  and  there  are  a  good  many 
species  which  live  in  fresh  water.  These  fresh- 
water forms  have,  as  a  rule,  but  little  of  the  red 


The  Lower  Plants  65 

pigment,  and  in  both  color  and  structure  show  a 
certain  resemblance  to  some  of  the  green  algae 
with  which  they  may  be,  perhaps,  remotely  related. 

Whatever  may  have  been  their  origin,  the  red 
algae,  as  they  now  exist,  are  very  highly  specialized 
plants  with  no  evident  relation  to  any  higher  plant 
types,  and  their  peculiarities,  including  the  character- 
istic red  pigment,  which  gives  them  their  name,  are 
presumably  adaptations  to  their  marine  environment. 

Reproduction  in  Algae. — The  algae  exhibit  great 
diversity  in  their  reproduction,  which  may  be  sexual 
or  asexual.  Several  forms  of  budding  or  simple 
fission  of  the  plant  body  are  common  and  in  a 
few  forms  it  is  the  only  reproduction  known. 

Thus  in  all  unicellular  species,  and  in  many  of 
the  lower  multicellular  forms,  the  individual  plant 
breaks  up  into  two  or  more  portions,  each 
of  which  becomes  at  once  a  new  individual. 
Very  often  special  reproductive  cells  are  formed, 
which  are  able  to  develop  without  fertilization 
into  new  plants.  The  commonest  of  these  non- 
sexual  reproductive  cells  are  the  zoospores.  In  the 
formation  of  these  zoospores  the  protoplasm  escapes 
from  a  cell,  either  in  a  single  mass  or  after  a  pre- 
liminary division  into  two  or  more  parts,  and  these 
on  escaping  into  the  water  are  seen  to  be  provided 
with  cilia  by  which  they  swim  rapidly  about  before 
they  settle  down  and  grow  into  new  plants.  In  their 
motile  condition  the  zoospores  so  closely  resemble 
the  low  organisms  known  as  Flagellata,  and  those 


66 


Plant  Life  and  Evolution 


curious  free-swimming  algae,  the  Volvocales,  that 
'hey  might  readily  be  mistaken  for  them. 

This   frequent   reversion  to  the   free-swimming 
condition,  resembling  in  all  respects  the  fully  devel- 


Evolutlon  of  the  sexual  reproduction  in  the  algae,  showing 
parallel  development  in  the  green  and  brown  algae.  Upper 
figures,  Chlorophyceae ;  lower  figures,  Phaeophycese ;  a,  female 
gamete;  b,  male  gamete. 

i,  Pandorina ;  2,  Aphanochaete ;  3,  CEdogonium ;  4,  Ectocar- 
pus;  5,  Cutleria;  6,  Fucus. 

Figs.  I — 5,  after  Oltmanns. 


oped  cells  of  the  lower  organisms,  is  one  of  the 
strongest  reasons  for  assuming  that  the  green  algae, 
at  least,  are  derived  from  flagellate  ancestors. 

Evolution  of  Sex  in  Algae. — Among    the    algae 
are  found  a  number  of  groups  in  which  are  exhib- 


The  Lower  Plants  67 

ited  in  a  most  instructive  fashion  the  gradual 
evolution  of  the  sexual  reproductive  cells.  It  is  clear 
from  a  study  of  these  that  this  evolution  has  arisen 
quite  independently  in  several  widely  separate  lines, 
but  the  course  of  evolution  is  extraordinarily  similar 
in  all  of  these.  The  Volvocales  and  the  Ulothri- 
cales,  among  the  green  algae,  are  excellent  exam- 
ples of  this,  and  the  brown  algae  also  show  all 
stages  of  evolution  of  the  gametes  or  sexual  cells, 
from  perfectly  similar  ones,  hardly  distinguishable 
from  the  non-sexual  zoospores,  to  clearly  differen- 
tiated small  male  cells  or  spermatozoids  and  large 
non-motile  female  cells  or  eggs  ( Fig.  5 ) . 

The  difference  between  the  lowest  type  of  sexual 
cells  and  the  non-sexual  zoospores  is  very  slight,  and 
there  seems  no  question  that  the  gametes  were  orig- 
inally derived  from  cells  capable  of  development 
without  fertilization.  Fertilization  in  its  simplest 
form  consists  in  the  union  of  two  complete  and  per- 
fectly similar  cells,  the  union  extending  to  the 
fusion  of  the  nuclei  into  one,  and  also  possibly  the 
fusion  of  the  chromatophores.  In  the  course  of  de- 
velopment the  two  gametes  become  more  and  more 
dissimilar,  one  diminishing  in  size  but  usually  re- 
maining actively  motile,  the  other  becoming  larger 
and  losing  the  power  of  motion.  The  first  is  the 
male  gamete  or  sperm,  and  is  largely  composed  of 
the  nucleus  of  the  mother  cell ;  the  large,  passive 
cell  is  the  female  cell,  egg,  or  ovum.  Among  the 
lower  types  the  gametes  may  sometimes  germinate 


68  Plant  Life  and  Evolution 

without  union,  thus  showing  but  little  difference 
from  the  non-sexual  zoospores.  Cases  are  known 
also,  even  among  some  of  the  more  specialized 
forms,  where  the  egg  will  develop  into  a  new  plant 
without  fertilization. 

Asexual  Algae. — Some  highly  developed  algae 
are,  apparently,  entirely  destitute  of  any  sexual  re- 
production. Until  recently  it  was  supposed  that  the 
giant  kelps  reproduced  themselves  by  non-sexual 
zoospores  only,  but  it  has  been  found  that  the  sup- 
posed zoospores  are  sometimes,  at  least,  gametes. 
So  also  the  curious  siphoneous  alga  Caulerpa, 
so  far  as  is  now  known,  multiplies  only  by  the  sep- 
aration of  a  portion  of  the  plant. 

In  the  fresh-water  green  algae  the  fertilized  egg 
usually  develops  into  a  thick-walled  "  spore "  or 
zygote.  This  is  capable  of  enduring  long  periods 
of  drought  and  is  presumably  a  provision  for  carry- 
ing the  plant  through  unfavorable  conditions,  espe- 
cially drought  or  cold.  These  resting  spores  almost 
never  are  formed  in  marine  plants,  as  the  latter 
never  are  subject  to  prolonged  periods  of  desicca- 
tion. The  zygote  on  germination  very  often  gives 
rise  to  a  number  of  zoospores,  by  a  division  of  its 
protoplasm,  and  thus  a  new  generation  starts  at  once 
with  several  individuals. 

In  the  red  seaweeds  there  is  a  marked  difference  in 
the  results  of  fertilization  from  that  found  in  the 
green  or  brown  algae.  Instead  of  the  female  cell 
developing  at  once  into  a  resting  spore,  it  is  at  once 


The  Lower  Plants  69 

stimulated  into  active  growth  and  gives  rise  either 
directly  or  indirectly  to  a  peculiar  structure  known 
as  the  "  Sporocarp,"  from  which  the  spores  are 
ultimately  developed.  The  development  of  the 
sporocarp  is  too  elaborate  to  be  given  here  in  detail, 
but  the  resulting  structure  may  be  compared  in  a 
way  to  the  spore-bearing  structure  or  "  sporophyte," 
which  arises  from  the  fertilized  egg  in  the  mosses 
or  ferns,  although  there  is  not  the  least  evidence  of 
any  relationship  between  the  latter  forms  and  the 
red  algae.  In  both  groups,  however,  the  product  of 
fertilization  is  a  spore-bearing  structure,  which  to 
a  greater  or  less  degree  is  parasitic  upon  the  plant 
which  bears  the  sexual  organs,  and  there  is  a  more 
or  less  well-marked  "  alternation  of  generations." 

Summary:  The  Fresh-water  Algae  More  Primi- 
tive than  the  Seaweeds — A  survey  of  the  Algae  as 
a  whole  indicates  that  the  main  line  of  develop- 
ment in  the  direction  of  the  higher  plants  is  through 
the  green  algae,  which  give  evidence  of  being  a  much 
more  primitive  group  than  either  the  brown  or  red 
algae.  There  is  a  fairly  complete  series  of 
forms  leading  from  the  free-swimming  Volvocales, 
through  non-motile  unicellular  forms,  to  simple 
filamentous  or  thallose  green  algae,  which  in  their 
turn  lead  toward  the  lowest  of  the  land 
plants,  the  simpler  mosses.  From  this  main  line 
there  probably  diverged  several  secondary  de- 
velopmental lines  such  as  the  Siphoneae  and 
Charales.  The  two  other  classes,  the  brown  and 


70  Plant  Life  and  Evolution 

red  algae,  are  forms  which  have  become  most 
perfectly  adapted  to  strictly  marine  life,  most  of 
the  more  primitive  green  algae  being  confined  to 
fresh  water.  Of  the  two  classes  of  seaweeds  the 
brown  algae  probably  constitute  a  separate  line,  de- 
rived from  some  flagellate  ancestors.  The  red 
algae,  while  different  in  many  ways  from  the  green 
algae,  still  among  their  lower  members  show  points 
of  resemblance  which  do  not  forbid  the  hypothesis 
that  they  may  have  arisen  from  forms  allied  to  some 
of  the  green  algae.  This  view  is  strengthened  by 
the  fact  that  a  good  many  of  the  more  primitive 
types  of  the  red  algae  inhabit  fresh  water.  The 
true  brown  algae  are  almost  exclusively  marine  in 
habit. 

If,  as  has  been  conjectured,  the  ancient  seas  were 
much  less  saline  than  those  of  the  present  time,  it 
may  be  that  the  green  algae  as  they  now  exist  are 
the  little  changed  descendants  of  the  primordial 
algal  types  which  have  persisted  in  fresh  water 
and  retained  most  of  their  original  characteristics. 
The  brown  algae,  so  far  as  we  can  judge,  are  essen- 
tially marine,  and  both  color  and  structure  may  be 
considered  to  be  direct  adaptations  to  the  marine 
life.  The  brown  pigment  is  assumed  to  be  pro- 
tective, as  these  plants  are  often  exposed  to  strong 
light  when  uncovered  by  the  tides,  and  the  leathery 
texture  and  gelatinous  tissues  of  the  larger  forms 
are  evidently  associated  with  their  growth  on  ex- 
posed rocky  shores. 


The  Lower  Plants  71 

THE  FUNGI 

The  algae  are  the  plants  which  may  be  consid- 
ered to  show  the  most  perfect  adaptation  to  aquatic 
life,  the  aquatic  ferns  and  seed  plants  being  proba- 
bly the  descendants  of  terrestrial  forms  which  have 
reverted  to  the  aquatic  habit.  There  are,  however, 
certain  plants  of  somewhat  heterogeneous  nature 
which  differ  very  widely  in  their  habits  from  the 
normal  plants  and  have  become  extraordinarily  mod- 
ified so  that  it  is  practically  impossible  to  trace  their 
ancestry.  These  are  the  Fungi,  which  include  such 
familiar  forms  as  mushrooms,  molds,  mildews,  rusts, 
an  enormous  assemblage  of  species,  second  in  num- 
ber only  to  the  flowering  plants.  The  Fungi  never 
possess  chlorophyll,  and  so  far  as  is  known  are  quite 
unable  to  assimilate  CO2  for  food,  and  hence  they 
are  dependent  upon  organic  matter  for  their  carbon, 
just  as  animals  are;  but  some  of  them,  like  the 
nitrogen  bacteria,  can  use  the  atmospheric  nitrogen. 

A  bit  of  bread  exposed  to  moist  warm  air 
soon  becomes  covered  with  a  growth  of  mold,  which 
an  examination  will  show  to  include  a  number  of 
quite  different  species.  These  arise  from  tiny  spores 
which  germinate  upon  the  moist  bread  and  quickly 
produce  a  tangled,  webby  mass  of  fine  threads  which 
ramify  through  the  bread,  breaking  down  the  starch 
by  means  of  the  ferments  or  enzymes  secreted  by 
the  invading  fungus  filaments,  and  using  the  starch 
for  food.  Similarly,  a  mushroom  growing  in  a 


72  Plant  Life  and  Evolution 

meadow  manured  by  the  animals  feeding  upon  the 
grass,  sends  its  filaments  deep  into  the  rich  earth, 
where  they  form  extensive  white  root-like  fibers 
which  attack  the  organic  matter  in  the  soil  much  as 
the  mold  attacks  the  starch  in  the  bread.  The  con- 
spicuous, umbrella-shaped  mushroom  is  merely  the 
fructification  of  the  plant,  most  of  whose  existence 
is  passed  under  ground.  Such  fungi  as  the  molds 
and  toadstools,  which  live  upon  dead  matter,  are 
known  as  saprophytes. 

Quite  different  in  their  habits  are  the  parasitic 
fungi,  which  attack  living  animals  and  plants,  and 
are  the  causes  of  many  of  the  most  serious  plant 
diseases.  A  common  animal  parasite  is  the  little 
fungus  that  often  kills  house  flies  in  the  autumn, 
and  causes  the  infected  insect  to  stick  to  a  window- 
pane,  where  it  is  surrounded  by  a  halo  of  tiny  spores 
shot  off  from  the  ends  of  the  filaments  that  protrude 
from  the  body  of  the  fly,  within  which  the  fungus 
has  finally  completed  its  work  of  destruction. 

A*familiar  vegetable  parasite  is  the  mildew  which 
so  often  appears  upon  rose  leaves,  distorting  them 
and  covering  the  diseased  area  with  a  gray  frost-like 
film.  In  this  case  the  parasite  lives  upon  the  surface 
of  the  host-plant,  and  simply  sends  little  suckers  into 
the  cells,  and  thus  obtains  the  necessary  food.  Other 
parasitic  fungi,  like  the  rusts,  live  within  the  body 
of  the  host-plant,  and  break  through  the  surface 
only  for  the  purpose  of  distributing  their  spores. 

An  examination  of  the  cells  of  a  fungus  shows 


The  Lower  Plants  73 

that  there  is  no  trace  of  the  chromatophores  that  give 
the  green  color  to  most  plants.  The  fungi  are  there- 
fore unable  to  perform  photosynthesis,  and  so  far  as 
we  know,  must  obtain  their  carbon  from  the  organ- 
ized carbon  compounds  of  other  plants  or  animals. 

Origin  of  Fungi. — It  is  generally  assumed  that 
the  fungi  are  descended  from  plants  containing 
chlorophyll,  although  this  is  not  universally  ad- 
mitted, and  it  is  conceivable  that  the  true  fungi  rep- 
resent a  series  of  forms  which  have  never  possessed 
chlorophyll.  A  comparatively  small  .number  of 
fungi,  usually  associated  under  the  name  of  Alga- 
fungi,  show  a  more  or  less  evident  resemblance  to 
certain  algae,  and  some  of  them  at  least  may  very 
safely  be  considered  to  be  of  algal  origin.  It  is  an 
open  question,  however,  whether  these  alga-fungi 
are  really  related  at  all  to  the  true  fungi,  or,  indeed, 
whether  they  all  are  related  among  themselves. 
There  is  some  reason  to  suppose  that  some  at  least 
of  the  true  fungi  are  really  derived  from  these  alga- 
fungi  ;  but  this  point  is  by  no  means  certain. 

Among  the  most  alga-like  of  the  fungi  are  the 
water  molds  (Fig.  6),  whose  resemblance  to  cer- 
tain algae,  both  in  structure  of  the  plant  and  its 
reproduction,  is  sufficiently  close  to  make  probable 
a  real  relationship  between  the  forms.  The  water 
molds  are  found  growing  upon  the  bodies  of 
dead  insects  or  other  animals  in  the  water,  and 
sometimes  are  parasitic,  attacking  young  fish  or 
older  ones  that  have  been  wounded.  The  slender, 


74 


Plant  Life  and  Evolution 


more  or  less  branched  tubular  filaments  have  no 
division  walls,  and,  except  for  the  absence  of  chloro- 
phyll, resemble  very  closely  a  common  green  alga, 
Vaucheria.  As  in  the  latter,  there  are  two  sorts  of 


FIG.  6 

A — Sexual  reproductive  organs  of  a  siphoneous  alga,  Vau- 
cheria, compared  with  those  of  a  water-mold,  Rhiphidium,  B. 
C — Zoosporangium  of  Vaucheria. 
D — Zoosporangium  of  a  water-mold,  Saprolegnia. 
B— After  Thaxter. 

reproductive  organs,  non-sexual  zoospores,  and  sex- 
ually produced  resting  spores. 

Eumycetes  or  True  Fungi. — The  40,000  or  more 
species  known  as  true  fungi,  or  Eumycetes,  present 
an  almost  hopeless  tangle  of  forms  which  is  just  be- 
ginning to  be  unraveled.  At  present  the  classifica- 


The  Lower  Plants  75 

tion  is  in  a  very  chaotic  condition,  although  much  is 
being  done  to  clear  up  some  of  the  most  puzzling 
questions  relating  to  their  development  and  affini- 
ties. We  may  for  convenience's  sake  divide  them 
into  two  classes,  Ascomycetes,  or  Sac-fungi,  and 
Basidiomycetes,  which  include  the  familiar  mush- 
rooms, puff-balls,  etc.  It  must  be  confessed,  how- 
ever, that  only  with  difficulty  can  a  good  many  forms 
be  brought  within  these  categories;  and  as  to  the 
relation  of  these  two  groups  to  each  other  and  to 
the  alga-fungi  there  is  much  difference  of  opinion. 

Sac-fungi,  Ascomycetes Among  the  simpler 

sac-fungi  are  many  species  known  popularly  as  mil- 
dews, which  are  often  troublesome  parasites  upon 
various  plants.  The  common  rose-mildew  ( Sphsero- 
theca)  is  one  of  the  best  known.  Another  closely 
related  species,  which  has  been  carefully  studied,  is 
common  on  the  dandelion.  The  body  of  the  fungus, 
or  "  mycelium  "  as  it  is  technically  called,  consists  of 
a  mass  of  slender  filaments  composed  of  rows  of 
long  cells,  which  form  a  film  over  the  surface  of  the 
leaf  on  which  it  is  growing.  Into  the  epidermal  cells 
of  the  leaf  are  sent  little  suckers  by  means  of  which 
the  fungus  feeds,  and  presently  it  sends  up  upright 
branches  into  the  air,  from  which  little  spores  are 
cut  off  in  rapid  succession  and  quickly  grow  into 
a  new  mycelium  if  the  conditions  are  favorable. 

Another  type  of  spore  is  produced  also  by  most  of 
the  mildews.  As  the  result  of  a  simple  form  of 
sexual  reproduction,  two  cells  unite  and  the  con- 


76  Plant  Life  and  Evolution 

tents  of  one  passes  into  the  other  and  fuses  with  it. 
The  fertilized  cell  is  stimulated  into  growth  and 
produces  finally  one  or  more  sac-shaped  cells,  the 
"  asci  "  or  spore  sacs,  within  which  are  produced 
usually  eight  spores.  All  of  the  typical  sac-fungi 
develop  sooner  or  later  spores  of  this  type, 
from  which  the  class  receives  its  name.  The 
spore  sacs  are  only  occasionally  exposed,  as  for 
instance  in  the  fungus  Exoascus,  which  causes  the 
disease  of  peach  trees  known  as  "  leaf  curl."  Usu- 
ally there  is  developed  a  protective  envelope  of  sterile 
tissue  so  that  the  spore  sacs  are  contained  in  a  defi- 
nite fruiting  body  or  "  sporocarp,"  which  may  reach 
considerable  dimensions  as  in  the  scarlet  cup-fungi 
which  are  sometimes  encountered  in  damp  woods. 

While  the  development  of  this  spore  fruit  may  be 
preceded  by  the  fertilization  of  a  definite  "  ascogo- 
nium,"  in  most  of  the  large  forms  like  the  cup- 
fungi  this  has  not  been  demonstrated,  and  a  true 
fertilization  is  probably  wanting. 

Basidiomycetes. — The  mushrooms,  toadstools, 
puff-balls,  and  rusts  represent  the  second  great  di- 
vision, or  class  of  the  true  fungi,  the  Basidiomy- 
cetes. In  the  large  forms,  such  as  the  mushroom, 
the  spore  fruits  arise  from  the  extensively  developed 
mycelium  in  the  earth  or  rotten  wood  upon  which 
the  fungus  is  growing.  The  spore  fruits  are  often 
of  large  size  and  characteristic  form,  and  upon  cer- 
tain portions  are  borne  the  spores.  In  the  mushroom 
they  arise  from  the  surface  of  the  "  gills,"  the 


The  Lower  Plants  77 

pendent  radiating  plates  upon  the  lower  side  of  the 
cap.  The  spores  are  borne  upon  swollen  club-shaped 
cells,  or  basidia,  and  each  spore  is  attached  to  a 
short,  slender  stalk  from  which  it  is  readily  detached. 

In  the  lower  types  of  Basidiomycetes,  such  as  the 
wheat-rust,  the  spore  fruits  are  much  less  definite 
in  form,  and  several  different  sorts  of  spores  are 
produced  in  the  course  of  the  plant's  development. 

Nuclear  fusions  occur  at  certain  times,  and  these 
fusions,  in  some  of  the  lower  Basidiomycetes,  per- 
haps represent  a  very  rudimentary  type  of  fertiliza- 
tion. The  relation  of  the  Basidiomycetes  to  the  sac- 
fungi  is  not  at  all  clear,  and  it  is  not  unlikely  that  the 
two  classes  are  not  related  at  all. 

Nutrition  of  Fungi. — The  fungi,  as  we  have  seen, 
differ  very  essentially  in  their  nutrition  from  the 
green  plants,  being  unable,  so  far  as  we  know,  to 
utilize  inorganic  matter,  except  in  the  case  of  nitro- 
gen, for  the  manufacture  of  organic  food.  Much 
remains  to  be  learned,  however,  about  the  nutrition, 
which  is  often  extremely  peculiar.  Many  fungi  feed 
upon  dead  substances  and  are  therefore  important 
agents  in  organic  decomposition.  Others  are  para- 
sites, and  often  show  extraordinary  specialization. 
A  very  remarkable  type  of  parasitism  is  that  known 
as  "  hetercecism,"  where  the  parasite  lives  on  more 
than  one  host.  This  is  the  case  in  many  rusts,  one 
of  the  commonest  cases  in  Eastern  America  being 
that  of  the  rust  Gymnosporangium,  which  produces 
the  conspicuous  galls  known  as  "  cedar  apples " 


78  Plant  Life  and  Evolution 

upon  species  of  Juniper.  Upon  these  cedar  apples 
in  the  spring  are  produced  great  masses  of  orange- 
yellow  spores  imbedded  in  a  soft  jelly.  From  these 
spores  arise  others  which  will  not  grow  upon  the 
cedar,  but  will  germinate  if  they  are  carried  to  the 
opening  leaves  of  a  thorn  or  crab  apple,  upon  which 
they  produce  a  fungus  growth  entirely  different 
in  appearance  from  that  upon  the  cedar.  In  little 
cup-shaped  receptacles  which  appear  later  upon  the 
leaves  of  the  thorn,  are  borne  chains  of  spores, 
which,  carried  back  to  the  cedar,  give  rise  to  a  new 
crop  of  cedar  apples.  What  is  the  meaning  of  this 
change  of  host  is  not  clear,  but  it  is  paralleled  by  the 
behavior  of  many  animal  parasites  like  Trichina  and 
the  liver-flukes. 

Symbiosis. — Many  fungi  live  in  a  more  or  less 
perfect  symbiotic  relation  with  other  plants.  The 
best-known  cases  are  those  of  the  Lichens,  which,  as 
is  well  known  to  the  botanist,  are  associations  of 
fungi,  usually  sac-fungi,  with  various  low  algae. 
If  we  examine  the  structure  of  a  lichen,  it  is  easy 
to  see  that  it  is  much  like  a  true  fungus,  but  en- 
meshed among  the  colorless  filaments  of  the  fungus 
are  colonies  of  green  cells,  which  a  close  examina- 
tion shows  to  be  unicellular  algae,  upon  which  the 
fungus  filaments  are  parasitic.  It  is  these  green 
cells,  imprisoned  in  the  tangle  of  fungus  filaments, 
which  give  the  greenish  tinge  to  the  lichen.  Ex- 
actly what  role  each  of  the  symbionts  plays  is  not 
entirely  clear.  The  fungus  is  undoubtedly  to  some 


The  Lower  Plants  79 

extent  parasitic  upon  the  alga  and  cannot  exist  with- 
out it.  On  the  other  hand,  the  alga  can,  and  not 
infrequently  does,  grow  quite  independent  of  the 
fungus.  However,  it  is  by  no  means  unlikely  that 
the  latter  furnishes  to  the  alga  certain  food  con- 
stituents, probably  including  nitrogen.  Moreover, 
the  fungus  conserves  water  in  such  a  way  that  the 
alga  cells  associated  with  it  are  able  to  grow  as 
they  could  not  do  if  they  were  exposed  to  the  air. 

A  good  many  flowering  plants,  especially  those 
which  are  deficient  in  chlorophyll  and  especially 
those  that  grow  in  humus  like  the  Indian-pipe  and 
certain  orchids,  have  associated  with  them  a  fungus 
which  in  some  way,  not  very  clearly  understood, 
furnishes  them  with  certain  food  constituents  from 
the  humus  in  which  the  plants  are  growing  and 
make  them  available  for  the  use  of  the  plants.  Re- 
cent studies  on  these  forms  have  shown  that  some- 
times the  fungi  possess  the  power  of  fixing  free 
nitrogen,  like  the  nitrogen  bacteria,  and  it  is  likely 
that  the  associated  symbiont  gets  the  benefit  of  this 
by  its  association  with  the  fungus,  as  well  as  ob- 
taining the  carbon  which  it  cannot  fix  for  itself  by 
photosynthesis.  Some  ferns  and  liverworts  also 
••show  this  symbiotic  association  with  fungi. 

Many  species  of  parasitic  fungi  must  be  of  com- 
paratively recent  origin,  as  they  are  restricted  to 
a  single  host,  which  in  many  cases  is  a  highly  spe- 
cialized flowering  plant  and  must  be  a  relatively 
recent  development. 


CHAPTER  IV 
THE  ORIGIN  OF  LAND  PLANTS 

THE  algae  seem  to  have  reached  their  culmina- 
tion in  such  great  marine  forms  as  the 
giant  kelps  of  the  Pacific.  These  highly  special- 
ized brown  seaweeds  and  the  very  peculiar  red 
algae  are  the  dominant  plants  of  the  ocean  at  the 
present  time,  and  have  evidently  best  solved  the 
problem  of  life  in  salt  water;  to  their  peculiar 
environment  are  no  doubt  due  their  most  marked 
characteristics.  There  is  little  reason  to  suppose  that 
any  higher  plant  types  have  arisen  from  either  the 
brown  or  the  red  algae,  although  it  has  been  sur- 
mised that  there  may  be  a  possible  connection  be- 
tween the  latter  and  certain  fungi. 

Green  Algae  the  Ancestors  of  the  Land  Plants. — 
The  green  algae,  on  the  other  hand,  probably  repre- 
sent the  remnants  of  the  primordial  vegetation 
which  have  persisted  in  fresh  water  without  any 
very  great  alteration  down  to  the  present  time.  It 
is  from  forms  allied  to  these  primitive  green  algae 
that  there  is  good  reason  to  suppose  the  first  land 
plants  arose. 

Except  for  differences  in  temperature,  conditions 
so 


The  Origin  of  Land  Plants  81 

of  life  in  fresh  water  are  very  uniform  everywhere, 
and  it  is  not  strange  therefore  that  the  range  of 
structure  exhibited  by  the  fresh-water  green  algae 
is  comparatively  slight.  Owing  to  the  density  of 
the  medium  in  which  they  live,  no  mechanical  or 
supporting  tissues  are  required,  as  they  are  entirely 
supported  by  the  water.  In  consequence  most  algae 
when  taken  from  the  water  collapse.  Moreover, 
owing  to  their  complete  submersion  there  is  no  loss 
of  water  from  evaporation,  and  the  cells,  therefore, 
are  not  protected  against  evaporation  and  water  is 
absorbed  by  all  of  the  superficial  cells. 

Fresh- water  Plants  Require  Protection  Against 
Desiccation. — However,  as  most  fresh-water  plants 
are  liable  to  be  destroyed  by  the  drying  up  of  the 
temporary  ponds  or  streams  in  which  they  live,  it  is 
necessary  to  provide  for  their  survival  through 
periods  of  drought  to  which  they  may  be  subjected. 
Marine  plants  never  being  exposed  to  prolonged 
drying  up,  although  they  may  be  uncovered  by  low 
tide  for  several  hours,  have  no  need  for  such  pro- 
tective devices,  and  hence,  resting  spores  are  rarely 
found  in  these  marine  forms.  Some  of  the  lowest 
plants,  like  certain  of  the  blue-green  algae  and  the 
common  "  Protococcus  "  forms,  may  be  completely 
dried  up  in  their  vegetative  condition,  remaining 
dormant  for  an  indefinite  period,  and  then  when 
water  is  supplied  to  them  promptly  revive  and  re- 
sume their  activity.  Moreover,  some  of  these  low 
forms,  as  well  as  others  of  higher  rank,  find  suf- 


82  Plant  Life  and  Evolution 

ficient  moisture  for  their  needs  upon  the  surface  of 
the  shaded  ground  or  the  sheltered  sides  of  tree 
trunks  and  walls.  They  readily  absorb  water  from 
the  moist  substratum  or  from  the  air,  and  this  is 
sufficient  for  their  normal  growth.  A  small  num- 
ber of  algae,  e.g.,  Botrydium  (Fig.  7),  regularly 


FIG.  7 

Botrydium,  a  terrestrial  alga  provided  with  roots  for  taking 
up  water.    Much  enlarged. 

grow  entirely  exposed  to  the  air  and  obtain  their 
water  from  the  earth  by  means  of  root-like  organs, 
thus  behaving  like  genuine  land  plants;  but  owing 
to  their  extreme  delicacy  the  period  of  growth  is 
usually  very  brief,  and  as  their  cells  are  not  ade- 
quately protected  against  loss  of  water  by  evapora- 
tion, they  can  only  reach  very  small  dimensions. 


The  Origin  of  Land  Plants  83 

Resting  Spores  of  Green  Algae. — Most  of  the 
green  algae  at  the  close  of  their  active  growing 
period  produce  some  form  of  resting  spore,  thick- 
walled  cells  which  can  survive  complete  desiccation, 
and  are  thus  able  to  carry  the  plant  over  periods  of 
drought.  While  a  small  number  of  algae,  like  the 
Botrydium  already  referred  to,  may  assume  a  more 
or  less  complete  aerial  habit,  this  is  exceptional,  and 
we  must  look  to  the  next  branch,  or  sub-kingdom, 
of  plants,  comprising  the  mosses  and  ferns,  for  the 
first  green  plants  which  may  be  considered  to  be 
normally  of  terrestrial  habit. 

The  First  Terrestrial  Plants. — The  first  invasion 
of  the  land  by  the  algal  ancestors  of  the  higher 
plants  must  be  regarded  as  a  most  momentous 
event  in  the  history  of  the  vegetable  kingdom. 
The  much  greater  range  of  conditions  on  land 
affords  far  greater  possibilities  for  structural 
variation,  and  this  is  amply  shown  in  the  future 
history  of  the  plant  kingdom.  The  higher  plants 
are  mainly  organisms  adapted  to  life  in  the  air, 
and  show  a  complexity  and  variety  of  struc- 
ture far  surpassing  that  of  the  largest  and  most 
specialized  of  the  seaweeds,  which  seem  to  have  at- 
tained the  limits  of  structure  possible  within  the 
range  of  their  strictly  aquatic  environment.  The 
mosses  and  ferns,  as  we  shall  see,  show  unmistakable 
evidence  of  their  derivation  from  aquatic  ancestors, 
and  indeed  all  of  these  forms  may  be  considered  to 
be  amphibious,  as  the  development  of  certain  phases 


84  Plant  Life  and  Evolution 

of  their  life  history  is  dependent  upon  the  presence 
of  free  water. 

Modifications  Due  to  Terrestrial  Habit. — When 
the  plant  exchanges  its  aquatic  environment  for 
life  on  land  it  must  undergo  some  radical  changes 
in  structure.  First  of  all  is  the  necessity  for  econ- 
omizing water,  as  it  is  no  longer  able  to  take  in 
water  at  all  points,  and  it  must  therefore  provide 
both  for  the  absorption  of  water  and  for  checking 
undue  loss  of  water  through  transpiration.  Hence 
have  arisen  the  special  water-absorbing  roots, 
and  the  protection  of  all  the  exposed  cell  walls 
by  a  cuticle  or  impervious  membrane,  which  ma- 
terially checks  the  escape  of  water  through 
evaporation. 

The  lowest  of  these  land  plants,  such  as  the  liver- 
worts and  mosses,  are  often  prostrate  in  their  habit 
and  do  not  assume  the  upright  position  common 
to  most  of  the  higher  plants.  This  prostrate  habit, 
which  implies  an  imperfect  development  of  me- 
chanical or  supporting  tissues,  recalls  the  behavior 
of  their  algal  ancestors.  Where  the  upright  position 
is  assumed,  it  involves  a  greater  or  less  development 
of  firm  tissues,  the  so-called  mechanical  or  support- 
ing tissues,  which  give  to  the  plant  sufficient  rigid- 
ity to  overcome  the  force  of  gravity.  With  the 
increasing  size  of  the  plant  comes  the  need  for  rapid 
transportation  of  water,  and  there  have  arisen  in 
response  to  this  need  the  characteristic  conducting 
tissues,  which  reach  their  highest  development  in 


The  Origin  of  Land  Plants  85 

the  so-called  "  vascular  "  plants,  the  ferns  and  flow- 
ering plants. 

The  First  Land  Plants  Allied  to  Liverworts. — 
Certain  liverworts  probably  resemble  pretty  closely 
the  first  land  plants.  These  are  small  plants  of  very 
simple  structure,  lying  flat  upon  the  ground,  to 
which  they  are  attached  by  delicate  roots.  Struc- 
turally these  liverworts  are  some  of  them  less 
complex  than  many  of  the  algae,  often  being  com- 
posed of  almost  perfectly  uniform  cells.  They  show 


FIG.  8 

A — A  simple  liverwort,  Ricciocarpus,  showing  the  small 
globular  sporophytes,  sp,  imbedded  in  the  thallus. 

B — A  large  liverwort,  Treubia,  having  leaf-like  organs;  the 
sporophyte  has  a  long  stalk  or  seta,  carrying  up  the  terminal 
spore-bearing  capsule. 

certain  resemblances  to  some  of  the  simple  algae, 
especially  the  order  known  as  the  Ulothricales, 
which  on  the  whole  come  nearer  to  the  liverworts 
than  do  any  other  algse.  It  must  be  admitted,  how- 


86  Plant  Life  and  Evolution 

ever,  that  the  gap  between  the  algse  and  the  mosses 
is  a  very  wide  one. 

Occasionally  liverworts  are  found  which  are  true 
water  plants,  such,  for  example,  as  Ricciocarpus 
(Fig.  8,  A),  which  grows  ordinarily  as  a  floating 
aquatic.  If  the  water  dries  up,  however,  the  liver- 
wort settles  upon  the  mud  and  grows  very  luxuri- 
antly, the  contact  with  the  earth  acting  apparently 
as  a  stimulus.  Roots  are  developed  penetrating  the 
mud  and  the  plant  assumes  quite  a  different  form 
from  that  of  the  floating  condition.  The  behavior 
of  this  liverwort  may  perhaps  illustrate  the  first 
step  in  the  development  of  the  higher  plants  from 
alga-like  aquatic  ancestors.  These  water  plants 
stranded  upon  the  mud  by  the  subsidence  of  the 
water  may  have  developed  roots  in  response  to  con- 
tact stimulus  of  the  solid  earth,  and  prolonged 
their  growing  period,  and  thus  may  have  inaug- 
urated the  line  of  land  plants  which  was  destined 
to  become  the  dominant  plant  type  of  the  future. 

Amphibious  Nature  of  the  Archegoniates. — The 
essentially  amphibious  nature  of  the  mosses  and 
ferns  is  best  shown  in  their  method  of  fertilization. 
If  we  examine  a  liverwort  like  that  shown  in  the 
figure,  we  shall  find  the  sexual  cells,  eggs,  and 
sperms  borne  in  organs  of  characteristic  structure. 
The  female  organ  (Fig.  9,  C)  is  multicellular  and 
usually  has  the  form  of  a  long-necked  flask,  which 
contains  the  egg-cell.  This  structure  is  called  the 
archegonium  and  is  remarkably  uniform  in  struc- 


The  Origin  of  Land  Plants  87 

ture  in  all  mosses  and  ferns,  which  are  hence  called 
the  "  Archegoniates."  The  male  organ,  or  anthe- 
ridium, is  also  multicellular  and  contains  many 
sperm  cells,  each  of  which  gives  rise  to  a  ciliate 


FIG.  9 

A — Section  of  the  antheridium  or  male  organ  of  a  liver- 
wort, Riccia. 

B — The  motile  male  gamete,  or  sperm. 

C — The  female  organ,  or  archegonium,  containing  the  fe- 
male gamete,  or  egg,  o. 

D — The  embryo-sporophyte  enclosed  in  the  enlarged  basal 
part  of  the  archegonium. 

sperm  much  like  that  of  certain  algae  (Fig.  9,  A,  B). 
Neither  archegonium  nor  antheridium  will  open 
unless  wet;  but  if  the  ripe  organs  are  covered 
with  water  they  will  promptly  open,  and  the  lib- 
erated sperms  then  swim  to  the  open  archegonium, 


88  Plant  Life  and  Evolution 

which  they  enter,  and  one  of  them  fertilizes  the  egg 
in  precisely  the  same  way  as  happens  in  the  per- 
manently aquatic  algae. 

After  the  egg  is  fertilized  it  becomes  invested  with 
a  cell-wall,  but  it  does  not  enter  a  resting  state  as 
is  the  case  with  the  green  algae.  Instead  of  this, 
the  egg  cell  grows  and  undergoes  repeated  cell- 
divisions,  so  that  a  cellular  body,  the  embryo,  re- 
sults (Fig.  9,  D).  The  embryo,  by  further  growth, 
finally  develops  into  a  plant  which  is  entirely  differ- 
ent in  appearance  from  the  one  which  bears  the 
archegonium,  and  which  is  called  the  gametophyte 
or  sexual  plant.  The  plant  developed  from  the  em- 
bryo does  not  become  independent,  but  remains  at- 
tached to  the  sexual  plant,  upon  which  it  lives  in  a 
parasitic  fashion.  Sooner  or  later  it  gives  rise  to 
special  reproductive  cells,  which  are  produced  by 
cell-division  and  are  hence  non-sexual  in  their  na- 
ture. These  are  called  "  spores,"  and  the  plant 
bearing  them  is  known  as  the  "  sporophyte,"  or  non- 
sexual  plant. 

Alternation  of  Generations. — The  sporophyte  as- 
sumes greater  and  greater  importance  in  the  course 
of  the  evolution  of  the  archegoniates,  while  the 
gametophyte  becomes  less  and  less  conspicuous. 
From  the  spores  produced  by  the  sporophyte  there 
arise  new  gametophytes.  This  alternation  of  the 
sexual  plant  or  gametophyte  with  the  neutral  one, 
or  sporophyte,  produced  as  the  result  of  fertilization, 
constitutes  the  frequently  discussed  "  alternation 


The  Origin  of  Land  Plants  89 

of  generations "  which  characterizes  all  of  the 
higher  plants. 

Evolution  of  the  Gametophyte  in  Bryophytes. — 
The  gametophyte  in  the  simpler  liverworts  is  not 
strikingly  different  from  some  of  the  algae,  and,  as 
we  have  seen,  may  be  a  delicate  prostrate  thallus, 
composed  of  almost  uniform  cells  except  for  hair- 
like  roots  or  rhizoids,  and  the  reproductive  organs. 
From  this  simple  type  of  gametophyte  there  may  be 
traced  several  diverging  structural  types.  The 
gametophyte  reaches  its  highest  degree  of  special- 
ization in  some  of  the  larger  mosses  where  there 
are  developed  leafy  shoots  of  considerable  size,  a 
foot  or  more  in  length  occasionally,  and  these  show 
a  specialization  of  the  tissues  which  may  be  almost 
compared  to  that  of  the  sporophyte  of  the  ferns. 
The  roots,  however,  never  assume  the  perfect  form 
found  in  the  ferns  and  seed-plants,  but  are  usually 
composed  of  a  single  cell.  The  mosses  often  depend 
only  to  a  limited  extent  upon  these  hair-like  roots 
to  supply  them  with  water,  absorbing  the  water 
readily  through  the  leaves  very  much  as  the  algae 
do.  Liverworts  and  mosses  together  form  the 
group  of  Bryophytes. 

The  apparent  inability  of  the  gametophyte  to  de- 
velop adequate  roots,  probably  accounts  for  its  fail- 
ure to  reach  dimensions  at  all  comparable  to  those 
of  the  sporophyte  of  the  higher  plants.  Moreover, 
none  of  the  large  gametophytic  structures  have  de- 
veloped a  mechanical  system  of  tissues  sufficient 


90  Plant  Life  and  Evolution 

to  enable  them  to  maintain  a  truly  upright  position. 
The  larger  species  are  either  prostrate,  as  we  have 
seen  in  many  large  liverworts,  or  the  upright  posi- 
tion is  maintained  by  the  shoots  being  densely 
crowded  and  thus  affording  mutual  support.  It 
must  be  remembered  that  the  gametophyte  of  the 
archegoniates  is  the  transformed  progeny  of  some 
strictly  aquatic  plant,  and  it  is  not  unlikely  that 
there  are  limits  beyond  which  such  a  type  cannot 
progress.  So  far  as  we  know,  the  higher  mosses 
represent  the  extreme  development  on  land  of  these 
originally  aquatic  organisms,  and  they  cannot  be 
said  to  have  solved  very  satisfactorily  the  problem 
of  the  development  of  a  plant  type  perfectly  adapted 
to  life  on  the  land. 

Evolution  of  the  Sporophyte. — The  further  evo- 
lution of  the  plant  kingdom  is  mainly  bound  up  with 
the  neutral  generation  or  sporophyte.  The  origin 
of  this  is  to  be  looked  for  in  the  zygote,  or  resting 
spore,  so  commonly  developed  in  the  green  algae, 
as  the  last  phase  of  their  life  history.  This  zygote 
may  be  said  to  represent  the  terrestrial  phase  of  the 
alga,  as  it  is  fitted  to  survive  drought,  and  thus  to 
carry  the  plant  over  from  one  growing  period  to 
another.  The  fact  that  the  zygote,  which  is  the 
morphological  equivalent  of  the  sporophyte  of  the 
mosses  and  ferns,  is  from  the  very  first  a  structure 
fitted  for  existence  outside  the  water,  must  be  borne 
in  mind  in  following  out  the  further  history  of  the 
evolution  of  the  higher  plants. 


The  Origin  of  Land  Plants  91 

In  those  algae  which  are  assumed  to  be  the  near- 
est relatives  of  the  archegoniates,  the  zygote  on 
germination  produces  by  division  of  its  contents 
several  spores  which  in  most  cases  are  motile  zo- 
ospores.  This  division  of  the  zygote's  contents  into 
several  spores,  each  of  which  produces  a  new  plant, 
gives  of  course  an  advantage  over  those  forms  in 
which  the  zygote  develops  at  once  into  a  single 
plant.  In  one  of  these  green  algae,  Coleochaete, 
there  is  a  material  growth  in  the  size  of  the  egg 
after  it  has  been  fertilized,  and  when  the  spore 
germinates  there  is  developed  a  comparatively  large 
multicellular  body  which  resembles  the  embryo- 
sporophyte  formed  in  the  liverwort,  and  is  the  near- 
est approach  to  this  structure  that  has  yet  been 
discovered  among  the  algae.  Whether  or  not  this 
resemblance  indicates  a  true  relationship  has  been 
much  discussed  and  is  still  not  satisfactorily  settled. 
It  is  pretty  evident,  however,  that  the  sporophyte  of 
the  first  archegoniates  must  have  been  derived  from 
some  structure  which  could  not  have  been  very  dif- 
ferent from  the  sporophyte  of  Coleochaete. 

A  study  of  the  evolution  of  the  sporophyte  in  the 
lower  existing  archegoniates  demonstrates  clearly 
the  course  of  development  leading  up  to  the  higher 
plant  types.  In  Riccia,  for  example,  the  globular 
mass  of  tissue  derived  from  the  growth  and  division 
of  the  egg  has  practically  all  of  its  cells  devoted 
to  spore  formation,  there  being  only  a  single  layer 
of  sterile  tissue  upon  the  outside  (Fig.  10,  A). 


92  Plant  Life  and  Evolution 

Each  of  the  inner  cells  divides  into  the  four  spores, 
a  constant  character  in  all  archegoniates.  These 
spores  are  usually  capable  of  resisting  drought,  and 
correspond  physiologically  to  the  single  zygote  of 
the  algae.  The  retention  of  the  embryo  sporophyte 
within  the  archegonium,  and  its  nourishment  at  the 
expense  of  the  gametophyte,  enable  it  to  prolong  its 
period  of  growth,  with  a  corresponding  ability  to 
increase  its  output  of  spores — a  great  advantage  to 
the  plant,  as  a  single  fertilization  thus  results  in  a 
very  much  increased  number  of  spores  as  compared 
with  the  simpler  algae. 

Even  in  the  simplest  sporophyte,  like  that  of 
Riccia,  a  small  amount  of  tissue  remains  sterile,  i.e., 
does  not  give  rise  to  spores.  In  all  the  other  types 
the  amount  of  sterile  tissue  is  very  much  increased, 
and  it  soon  begins  to  develop  into  special  structures, 
indicative  of  a  division  of  labor,  and  this  involves 
a  much  longer  growing  period  for  the  developing 
sporophyte.  In  most  liverworts  (Fig.  10,  B)  the 
lower  part  of  the  embryo  is  very  early  separated 
from  the  upper  portion  from  which  the  spores  are 
developed,  and  this  lower  part  becomes  a  definite 
organ  of  absorption,  the  foot,  by  means  of  which 
food  is  taken  from  the  tissues  of  the  gametophyte 
for  the  nutrition  of  the  embryo-sporophyte,  which 
thus  may  be  said  to  live  parasitically  upon  the 
parent  gametophyte.  Other  structures  may  also 
develop  from  the  sterile  tissue,  such  as  the 
elongated  seta  or  stalk,  which  may  reach  a  con- 


The  Origin  of  Land  Plants 


93 


siderable  length,  and  facilitates  the  disposal  of  the 
spores  developed  in  the  upper  region  of  the  sporo- 
phyte  (Fig.  8,  B).  The  latter  forms  a  capsule  en- 


FlG.  10 

Diagrams  illustrating  the  evolution  of  the  sporophyte  in  the 
liverworts.  The  shaded  areas  show  the  extent  of  the  sporog- 
enous  tissue ;  f,  the  foot. 

A — Riccia. 

B— Porella. 

C — Anthoceros. 

closing  the  spores,  together  with  peculiar  sterile 
cells,  or  elaters,  formed  from  a  portion  of  the  po- 
tentially sporogenous  tissue. 


94  Plant  Life  and  Evolution 

Subordination  of  Spore  Production  in  the  Higher 
Archegoniates. — Among  the  liverworts  proper,  the 
function  of  the  sporophyte  is  almost  entirely  spore 
production,  and  it  develops  little  or  no  chlorophyll, 
so  that  it  is  capable  of  very  little  independent 
growth.  In  two  other  classes  of  the  bryophytes, 
viz.,  the  true  mosses  and  the  horned  liverworts 
(Anthocerotes),  the  sporophyte  becomes  much  more 
important  and  spore  production  is  to  a  considerable 
extent  subordinated  to  the  vegetative  life  of  the 
sporophyte.  In  these  forms  the  growing  period  of 
the  sporophyte  may  last  for  several  months  before 
the  spores  are  finally  developed,  and  only  a  relatively 
small  portion  of  the  sporophytic  tissue  gives  rise  to 
spores  (Fig.  10,  C).  A  large  amount  of  green  tis- 
sue is  present  in  the  outer  portion  of  the  sporophyte, 
and  this  may  form  a  spongy  green  tissue  quite  like 
that  in  the  leaves  of  the  higher  plants.  As  in  the 
latter,  this  green  assimilative  tissue  may  communi- 
cate with  the  atmosphere  by  means  of  special  pores 
or  stomata,  which  structurally  closely  resemble  those 
of  the  vascular  plants.  By  means  of  these  well  de- 
veloped chlorophyll-bearing  tissues,  the  sporophyte 
can  assimilate  the  CO2  of  the  atmosphere,  and  it  is 
quite  independent  of  the  gametophyte  for  its  supply 
of  organic  food.  In  some  of  the  true  mosses  the 
green  tissue  is  largely  segregated  at  the  base  of  a 
capsule,  where  it  forms  a  sort  of  assimilative  organ 
known  as  the  apophysis.  This  may  be  said  to  take 
the  place,  physiologically  at  least,  of  a  leaf.  There 


The  Origin  of  Land  Plants  95 

are  also  present  in  many  of  the  true  mosses,  ex- 
tremely specialized  structures  associated  with  the 
opening  of  the  sporogonium  and  the  scattering  of 
the  spores.  These  very  highly  differentiated  struc- 
tures indicate  that  the  true  mosses  constitute  a 
very  specialized  class  with  little  direct  affinity  with 
any  other  plants.  They  may  be  said  to  bear  some- 
what the  same  relation  to  the  relatively  primitive 
liverworts  that  red  and  brown  seaweeds  do  to  the 
more  primitive  green  algae. 

Anthocerotes. — There  is  one  family  of  liverworts, 
now  usually  considered  to  represent  a  distinct  class, 
which  is  especially  important  in  the  study  of  the 
evolution  of  the  sporophyte.  These  are  the  horned 
liverworts  (Anthocerotes),  which  in  the  character 
of  their  sporophyte  approach  more  nearly  the  con- 
dition found  in  the  ferns  than  is  the  case  with  any 
other  bryophyte.  The  gametophyte  of  these  An- 
thocerotes is  very  simple  in  structure,  and  in  the 
character  of  the  cell  which  contains  only  a  single 
chloroplast  they  resemble  the  green  algae  more 
nearly  than  do  any  other  liverworts. 

Sporophyte  of  Anthoceros. — It  is  the  sporophyte, 
however,  with  which  we  are  here  especially  con- 
cerned. In  its  most  highly  developed  form,  i.e., 
Anthoceros  (Fig.  10,  C),  it  shows  a  remarkable 
power  of  growth.  There  is  developed  a  basal  zone 
of  growing  tissue,  which  keeps  adding  to  the  size  of 
the  sporophyte  so  that  sometimes  it  may  reach  the 
length  of  ten  centimeters  or  more.  To  support  this 


96  Plant  Life  and  Evolution 

long-continued  growth,  a  very  large  bulbous  foot  is 
developed,  and,  although  this  has  no  direct  connec- 
tion with  the  earth,  there  may  sometimes  be  seen 
to  be  an  extraordinary  development  of  roots  from 
the  under  side  of  the  gametophyte  immediately  be- 
low it,  and  this  unusual  development  of  the  roots 
is  obviously  induced  by  the  protracted  growth  of 
the  sporophyte,  and  its  increased  need  for  a  greater 
supply  of  water.  The  large  sporophyte  becomes 
almost  independent  of  the  gametophyte,  but  not 
wholly  so,  as  it  is  still  dependent  upon  it  for  its 
supply  of  water.  The  upper  part  of  the  sporophyte 
shows  a  well-developed  epidermis,  perforated  by 
numerous  stomata,  exactly  like  those  of  the  higher 
plants.  Beneath  the  epidermis  are  several  layers  of 
green  cells  with  intercellular  spaces  communicating 
with  the  openings  of  the  stomata,  and  this  green  tis- 
sue, both  in  structure  and  function,  closely  resembles 
the  mesophyll  or  green  tissue  of  an  ordinary  leaf. 

The  axis  of  the  sporophyte  is  occupied  by  a 
strand  of  much  elongated  cells,  which  are  presuma- 
bly more  or  less  active  agents  in  the  conduction  of 
water,  and  possibly  may  be  regarded  as  the  equiva- 
lent of  the  vascular  bundle,  which  occupies  a  similar 
position  in  the  young  organs  of  the  sporophyte  in 
the  so-called  "  vascular "  plants.  In  Anthoceros, 
the  spore-producing  tissue  is  reduced  to  a  single 
layer  of  cells  situated  below  the  green  assimilative 
tissue.  There  is  sometimes  a  more  or  less  complete 
separation  of  this  tissue  into  fertile  and  sterile  areas, 


The  Origin  of  Land  Plants  97 

the  former  being  enclosed  in  the  meshes  of  a  net- 
like  complex  of  sterile  cells.  This  segregation  of  the 
sporogenous  cells  into  groups  is  probably  the  first 
hint  of  the  definite  spore-bearing  organs  or  spo- 
rangia, which  are  characteristic  of  the  ferns.  Were 
the  sporophyte  of  Anthoceros  to  develop  a  true  root, 
i.e.,  to  come  into  direct  contact  with  the  source  of 
water  supply  and  soil  constituents,  the  sporophyte 
would  be  rendered  quite  independent,  since  the 
highly  developed  photosynthetic  apparatus  is  ample 
to  provide  for  the  assimilation  of  CO2. 

Whether  or  not  the  Anthocerotes  are  considered 
to  be  directly  related  to  the  ancestors  of  the  Pterido- 
phytes,  or  ferns,  there  is  no  question  that  both  in 
the  character  of  the  reproductive  organs  and  that  of 
the  sporophyte,  they  resemble  more  nearly  the  Pteri- 
dophytes  than  do  any  other  liverworts. 

Sporophyte  First  Becomes  Independent  in  the 
Ferns. — Although  in  Anthoceros  and  the  higher 
mosses,  the  sporophyte  attains  a  large  measure  of 
independence,  it  never  becomes  entirely  independent 
of  the  gametophyte  upon  which  it  must  draw  for  its 
water  supply,  owing  to  the  failure  to  make  direct 
connection  with  the  earth.  In  the  second  division 
of  the  Archegoniates,  on  the  other  hand,  the 
Pteridophytes  or  Ferns,  the  young  sporophyte  at  an 
early  period  develops  a  true  root  (Fig.  n,  A,  r), 
which  pierces  the  tissues  of  the  gametophyte  and 
grows  downward  into  the  ground,  so  that  the  young 
sporophyte  henceforward  absorbs  its  water  supply 


98  Plant  Life  and  Evolution 

directly  from  the  earth,  and  the  sporophyte  for  the 
first  time  assumes  the  form  of  an  entirely  inde- 
pendent plant.  For  a  greater  or  less  time,  however, 
it  remains  attached  to  the  gametophyte  and  develops 


FIG.  ii 

A — Gametophyte,  g,  of  a  fern,  Danaea,  bearing  two  young 
sporophytes,  sp.  Each  sporophyte  has  produced  a  leaf  above 
and  a  root,  r,  below. 

B — Gametophyte  of  a  liverwort,  Megaceros,  with  two  sporo- 
phytes ;  the  latter,  unlike  those  of  the  fern,  have  no  root. 

a  foot  very  similar  in  structure  and  function  to  that 
found  in  the  sporophyte  of  the  bryophytes. 

The  Gametophyte  of  the  Fern. — If  the  spores  of 
a  fern  are  sown  upon  moist  earth,  there  will  pres- 
ently be  developed  a  little  green  thallus,  the  gameto- 
phyte (Fig.  ii,  A,  g),  which  does  not  in  the  least 
resemble  a  fern,  but  in  appearance  is  very  much 
like  some  of  the  simpler  liverworts,  and  has  its  near- 
est analogue  perhaps  in  the  horned  liverworts,  whose 
reproductive  organs,  both  antheridium  and  arche- 


The  Origin  of  Land  Plants  99 

gonium,  show  some  interesting  resemblances  in 
structure  to  those  of  the  lower  ferns.  The  arche- 
gonium  and  antheridium  in  all  their  essential  fea- 
tures are  very  similar  to  those  in  the  liverworts  and 
mosses,  and,  as  in  those,  water  is  essential  both  for 
the  opening  of  the  ripe  reproductive  organs  and  for 
the  conveyance  of  the  motile  sperms.  Its  de- 
pendence upon  water  and  the  presence  of  these  mo- 
tile sperms  clearly  indicate  the  aquatic  origin  of  the 
gametophyte. 

There  is  much  difference  among  the  different 
groups  of  pteridophytes  in  the  degree  of  the 
development  of  the  gametophyte  and  as  might  be 
expected,  those  forms  which  for  other  reasons 
may  be  assumed  to  be  the  oldest  and  most  primi- 
tive types,  are  those  in  which  the  gameto- 
phyte is  most  important  and  most  nearly  resembles 
that  of  the  hypothetical  liverwort-like  progenitors. 
In  some  of  the  more  primitive  ferns  (Fig.  n,  A), 
the  gametophyte  may  reach  an  inch  or  so 
in  length  and  may  live  for  several  years,  not 
always  dying  after  the  sporophyte  has  become  estab- 
lished. In  some  of  the  more  specialized  ferns,  on 
the  other  hand,  like  the  so-called  water  ferns,  the 
whole  life  of  the  gametophyte  may  extend  over  less 
than  twenty-four  hours. 

In  their  fertilization  and  early  development  of 
the  embryo-sporophyte,  the  ferns  closely  resemble 
the  simpler  liverworts,  but  sooner  or  later  there 
is  evident  the  development  of  special  organs, 


ioo  Plant  Life  and  Evolution 

which  are  absent  from  the  embryo  of  the  liverworts. 
There  is  soon  formed  an  outgrowth  which  breaks 
through  the  overlying  tissues  of  the  gametophyte 
and  expands  itself  in  the  light  as  a  little  fan-shaped 
green  leaf.  Another  outgrowth  pushes  downward 
and  penetrates  the  soil  and  forms  the  primary  root, 
while  a  third  structure  becomes  the  permanent  grow- 
ing point,  or  stem  apex  of  the  little  sporophyte, 
which  is  now  recognizable  as  the  young  fern  (Fig. 
n,  A).  During  its  early  development,  the  little 
sporophyte  draws  its  nourishment  from  the  game- 
tophyte by  means  of  a  foot  quite  like  that  found  in 
the  mosses  and  liverworts,  but  so  soon  as  the  first 
leaf  is  expanded,  and  the  primary  root  has  pene- 
trated the  earth,  the  young  sporophyte  becomes  an 
independent  plant. 

In  the  ferns  the  development  of  spores  is  often 
delayed  for  many  years,  the  sporophyte  in  the  mean- 
time increasing  in  size  and  developing  special  organs 
and  tissues  which  characterize  these  "  vascular " 
plants.  The  young  sporophyte  soon  shows  a  definite 
axis  or  stem,  which  usually  possesses  a  permanent 
growing  point,  from  which  arise  later  on  many 
leaves  and  roots.  An  extensive  system  of  conduct- 
ing tissue  is  a  characteristic  of  the  sporophyte  of  the 
ferns,  which  are  called  sometimes  for  this  reason 
"  vascular  cryptogams."  This  "  fibro-vascular  " 
system  constitutes  a  tissue  especially  modified  for 
water  conduction  and  for  the  transport  of  food  sub- 
stances. While  similar  tissues,  as  we  have  seen,  may 


The  Origin  of  Land  Plants         101 

be  found  in  some  of  the  large  algae,  like  the  great 
kelps,  and  also  in  various  mosses,  they  never  reach 
the  perfect  development  that  distinguishes  them  in 
the  sporophytes  of  the  ferns  and  seed-plants. 

The  Root  of  Pteridophytes. — With  the  develop- 
ment of  the  primary  root,  which,  unlike  the  hair-like 
"  rhizoids  "  of  the  gametophyte,  is  a  massive  struc- 
ture capable  of  extensive  growth  and  admirably 
fitted  for  the  absorption  of  the  water  from  the  soil 
and  its  transportation  to  the  different  organs  of 
the  sporophyte,  the  independence  of  the  sporophyte 
is  for  the  first  time  perfectly  established;  and  we 
have  henceforth  to  deal  with  plants  which  are  not 
modifications  of  an  originally  aquatic  type,  as  is  the 
case  with  the  gametophyte,  but  are  elaborations  of 
a  structure,  the  zygote  or  resting  spore  of  the  algae, 
which  is  from  the  beginning  a  structure  adapted 
to  terrestrial  conditions.  This  probably  accounts 
for  the  perfect  adaptation  of  the  sporophyte  to  ter- 
restrial life,  when  compared  with  the  indifferent  suc- 
cess as  land  plants  of  even  the  most  perfect  gameto- 
phytic  structures,  like  those  of  the  larger  mosses. 
The  latter,  owing  to  their  failure  to  develop  an  ade- 
quate root  system,  seem  to  have  about  exhausted 
the  possibilities  of  the  aquatic  gametophyte,  and  the 
further  development  of  the  vegetable  kingdom  is 
mainly  bound  up  with  the  amplification  of  the  ter- 
restrial sporophyte.  With  the  inauguration  of  this 
entirely  new  plant  type  begins  the  most  important 
chapter  in  the  history  of  the  vegetable  kingdom. 


102 


Plant  Life  and  Evolution 


The  Sporophyte  of  the  Fern  a  Long-lived  Plant. 
— The  sporophyte  of  the  fern,  unlike  that  of  the 


~  I  yg? 


FIG.  12 

Diagrams  to  show  the  possible  method  of  evolution  of  the 
sporangia  of  the  lower  ferns. 

A — Section  of  the  upper  part  of  the  sporophyte  of  Antho- 
ceros,  a  liverwort  in  which  the  sporogenous  tissue  is  im- 
perfectly divided  into  fertile  areas  by  the  intervention  of 
sterile  cells;  these  are  unshaded  in  the  diagram. 

B — Section  of  the  sporangiophore  of  a  very  simple  fern, 
Ophioglossum.  The  originally  continuous  sporogenous  tissue 
becomes  divided  into  distinct  masses  of  fertile  tissue,  sep- 
arated by  wide  partitions  of  sterile  tissue;  each  sporangium 
opens  separately. 

C — Cross-section  of  B. 

D — Sporangia  of  Botrychium,  a  fern  closely  related  to  Ophi- 
oglossum, but  having  much  better  differentiated  sporangia, 
each  opening  by  a  transverse  slit. 

mosses  and  liverworts,  is  not  limited  in  its  growth, 
and  its  life  does  not  end  with  the  dispersal  of  the 
ripe  spores.  It  may  even  live  for  many  years  and 


The  Origin  of  Land  Plants         103 

attain  very  imposing  dimensions,  some  of  the  tree- 
ferns  rivaling  the  palms  in  size  and  beauty.  The 
original  stem  apex  often  persists,  but  new  leaves 
and  roots  are  produced  to  meet  the  needs  of  the  de- 
veloping sporophyte,  which  may  live  for  a  century 
or  more.  Sooner  or  later  spores  are  produced,  the 
development  following  very  close  that  noted  for  the 
liverworts  and  mosses;  but  in  the  ferns  the  spores 
are  borne  in  special  organs  or  sporangia,  which  in 
the  simpler  types  like  the  adder-tongue  ferns  (Fig. 
12,  B,  C)  suggest  somewhat  the  condition  found  in 
Anthoceros. 

The  curious  horsetails  or  scouring  rushes  com- 
mon in  low  ground  and  moist  thickets  represent  a 
second  class  of  pteridophytes  which  differ  strikingly 
in  their  habits  from  the  ferns.  The  hollow- jointed 
stems  with  the  leaves  reduced  to  toothed  sheaths  en- 
circling the  joints,  together  with  the  characteristic 
cones  which  bear  the  spore  cases,  at  once  distinguish 
the  horsetails  from  the  other  pteridophytes  (Fig.  13, 
C,  D).  The  elongated,  pendent  sacs,  or  sporangia, 
containing  the  spores,  are  arranged  about  the  mar- 
gins of  the  umbrella-shaped  "  sporangiophores " 
which  make  up  the  terminal  cones.  In  their  struc- 
ture and  development  the  sporangia  are  not  essen- 
tially different  from  those  of  the  lower  ferns. 

About  twenty-five  species,  all  belonging  to  the 
genus  Equisetum,  are  all  that  are  known  to  exist 
at  the  present  day,  the  insignificant  remnant  of  the 
many  large  and  complex  horsetails  that  flourished  in 


104  Plant  Life  and  Evolution 

earlier  geological  times.  Unlike  the  ferns  as  these 
plants  are  in  their  general  habit,  they  nevertheless 
show  many  points  of  resemblance  to  the  lower  ferns 
in  their  characters,  both  of  gametophyte  and  sporo- 
phyte,  and  it  is  probable  that  there  is  a  real,  although 
remote  relationship  existing  between  the  horsetails 
and  ferns. 

In  the  Northern  forests  one  often  finds  covering 
the  ground,  evergreen  trailing  plants  whose  slender 
creeping  stems  send  up  little  branching  shoots  closely 
set  with  small  leaves,  so  that  they  suggest  little 
evergreen  trees.  These  are  "  club-mosses  "  belong- 
ing to  the  genus  Lycopodium,  and  one  of  them, 
popularly  known  as  "  ground-pine,"  is  every  year 
sent  in  great  quantities  to  the  Northern  markets  at 
Christmas  time  for  making  wreaths  and  festoons. 
These  club-mosses,  or  Lycopods,  represent  a  third 
class  of  pteridophytes,  which  differ  so  much  from 
the  ferns  and  horsetails  as  to  make  it  likely  that 
they  are  not  at  all  related  to  them,  but  have  orig- 
inated quite  independently  (Fig.  13,  A). 

A  second  genus,  Selaginella,  contains  several  hun- 
dred species,  a  few  of  which  occur  in  temperate 
regions,  but  most  of  them  are  tropical.  A  num- 
ber of  species  are  common  in  greenhouses,  where 
their  delicate  moss-like  foliage  is  very  ornamental. 
These  cultivated  species  are  often  erroneously  called 
Lycopodium,  but  may  be  distinguished  from  the 
true  Lycopodiums  by  having  two  sorts  of  spores, 
large  and  small.  In  the  character  of  its  spores, 


The  Origin  of  Land  Plants         105 

Selaginella  comes   nearer  the   seed-bearing  plants 
than  does  any  other  living  pteridophyte. 

Paleozoic    Pteridophytes. — The    horsetails    and 
club-mosses  were  once  very  much  better  developed 


FIG.  13 

A — A  club-moss,  Lycopodium,  showing  the  spore-bearing 
cone,  or  strobilus,  sp. 

B — A  sporophyll  with  its  single  sporangium. 

C — A  horsetail,  Equisetum,  showing  the  sheath-like  leaves, 
v,  and  the  terminal  cone  made  up  of  the  sporangiophores,  a 
single  one  of  which  is  shown  in  D — sp,  sporangia. 

than  at  the  present  time,  this  being  especially  the 
case  in  the  Paleozoic  types.  Many  of  the  Paleozoic 
pteridophytes  reached  tree-like  dimensions  and 
showed  a  secondary  growth  in  thickness  of  their 
stems  comparable  to  that  which  is  found  in  most 


106  Plant  Life  and  Evolution 

living  trees.  This  secondary  thickening  occurs  in 
a  slight  degree  in  a  few  living  ferns,  and  it  is  clear 
that  this  peculiar  development  has  arisen  quite  in- 
dependently in  all  of  the  main  groups  of  pterido- 
phytes,  but  has  been  lost  in  most  of  the  de- 
scendants of  the  Paleozoic  pteridophytes.  In  the 
club-mosses  there  are  developed  minute  sperms  with 
two  cilia  like  those  of  the  mosses,  while  in 
the  ferns  and  horsetails  the  sperms  are  very  much 
larger  and  have  numerous  cilia,  and  do  not  at  all 
resemble  the  sperms  of  any  known  bryophyte. 

While  in  the  lower  types  of  the  pteridophytes, 
like  some  species  of  Lycopodium  and  the  lower 
ferns,  the  gametophyte  may  reach  a  size  comparable 
with  that  of  many  liverworts,  in  the  more  special- 
ized types  there  is  a  great  reduction  in  the  size 
of  the  gametophyte  which  may  live  a  very  short 
time. 

A  peculiar  modification  of  the  gametophyte  is 
sometimes  met  with.  This  is  the  development  of 
a  subterranean  habit,  with  the  entire  loss  of  chloro- 
phyll. The  gametophyte  thus  becomes  saprophytic, 
living  upon  the  leaf  mold  or  the  soil  rich  in  organic 
matter.  To  enable  it  to  do  this  it  has  associated 
itself  with  a  fungus,  much  like  that  found  in 
the  subterranean  organs  of  certain  saprophytic  flow- 
ering plants.  In  some  way,  not  perfectly  under- 
stood, it  is  through  the  agency  of  this  "  endophyte  " 
supplied  with  carbon  and  perhaps  also  with  nitrog- 
enous matter. 


The  Origin  of  Land  Plants          107 

Heterospory. — In  the  horsetails,  and  in  certain 
ferns,  the  gametophyte  is  unisexual,  the  male  plants 
being  usually  smaller  than  the  female,  which  of 
course  has  to  support  the  young  sporophyte.  This 
separation  of  the  sexes  is  the  first  step  in  the  direc- 
tion of  what  is  known  as  "  heterospory,"  i.e.,  the 
production  of  two  sorts  of  spores,  large  and  small, 
developing  respectively  female  and  male  gameto- 
phytes.  The  large  spores  are  known  as  macrospores 
or  megaspores,  the  small  ones,  microspores.  In  all 
cases  of  heterospory,  the  gametophytes  are  much 
reduced  in  size,  this  being  especially  true  of  the 
males,  which  may  consist  of  a  few  cells  only,  and 
have  their  whole  development  completed  within  less 
than  twenty-four  hours.  The  most  marked  case  of 
this  is  shown  in  the  water  fern,  Marsilia.  In 
this  plant  only  one  spore  comes  to  maturity  in 
the  megasporangium,  while  in  the  microsporangium 
all  of  the  spores  mature.  If  the  ripe  spores  are 
placed  in  water  of  a  suitable  temperature,  growth 
begins  at  once,  and  in  a  common  Calif ornian  spe- 
cies, within  from  fifteen  to  twenty  hours  the  gameto- 
phytes are  completely  developed  and  fertilization  has 
been  effected.  The  spermatozoid  fertilizes  the 
archegonium,  and  from  the  egg  the  young  sporo- 
phyte, which  closely  resembles  that  of  the  typical 
ferns,  is  quickly  developed. 

Heterospory  has  arisen  in  several  quite  unrelated 
groups  of  Pteridophytes,  and  it  is  clear  from  a  study 
of  the  fossil  forms  that  a  number  of  the  hetero- 


io8  Plant  Life  and  Evolution 

sporous  types  which  once  existed  are  now  quite 
extinct.  This  tendency  towards  heterospory  in  so 
many  unrelated  groups  is  an  interesting  case  of 
parallel  development,  and  may  be  compared  to  the 
separate  evolution  of  sexual  cells  in  widely  separate 
groups  of  algse,  or  the  development  of  a  similar 
fibre-vascular  system  in  such  probably  unrelated 
types  as  the  ferns  and  club-mosses.  There  are  four 
quite  distinct  existing  families  of  heterosporous 
pteridophytes,  two  of  which  show  unmistakable  evi- 
dence of  being  derived  from  two  types  of 
"  homosporous  "  ferns.  One  of  the  others  is  unmis- 
takably related  to  the  club-mosses;  the  second  is  of 
somewhat  doubtful  affinity. 

The  evolution  of  heterospory  in  so  many  unre- 
lated groups  of  pteridophytes  is  of  importance  in 
connection  with  the  question  of  the  origin  of  seeds, 
the  structures  which  especially  distinguish  the  high- 
est plant  types.  The  separation  of  the  male  and 
female  gametophytes,  and  the  reduction  of  these,  are 
the  first  steps  in  the  evolution  of  the  seed.  In  all 
of  the  existing  pteridophytes,  except  the  genus  Se- 
laginella,  the  germination  of  the  spore  and  the  de- 
velopment of  the  gametophyte  takes  place  after  the 
spores  are  shed,  although  it  may  be  said  that  in 
some  of  the  water- ferns  the  spores  do  not  become 
free  from  the  sporangium,  which  is  detached  with 
the  spores  inside  it.  In  some  of  the  water  ferns  the 
reduction  of  the  female  gametophyte  is  less  than 
in  the  other  heterosporous  forms  and  chlorophyll 


The  Origin  of  Land  Plants         109 

is  developed,  so  that  the  gametophyte  is  to  some 
extent  self-supporting,  while  in  other  forms  the 
growth  is  either  entirely  at  the  expense  of  material 
stored  up  in  the  spore,  or  in  Selaginella,  as  we 
shall  presently  see,  the  gametophyte  is  nourished 
directly  from  the  tissues  of  the  sporophyte,  thus  re- 
versing the  relation  of  sporophyte  to  gametophyte 
as  compared  with  the  lower  archegoniates. 

In  Selaginella  germination  takes  place  in  the 
megaspore  while  it  is  still  in  the  sporangium,  and 
long  before  it  has  reached  its  full  size.  At  this  time 
the  spore  contains  but  little  protoplasm,  and  the  sub- 
sequent growth  of  the  gametophyte,  which  until  it 
is  nearly  fully  grown  is  included  entirely  in  the 
spore,  is  mainly  at  the  expense  of  the  sporophyte. 
The  cells  of  the  sporangium  walls  remain  active, 
and  through  them  material  is  drawn  from  the  sporo- 
phyte for  the  nourishment  of  the  developing  gameto- 
phyte inside  the  growing  spore.  The  gametophyte 
may  be  said,  therefore,  to  grow  parasitically  upon 
the  sporophyte,  thus  reversing  the  conditions  which 
obtain  in  the  lower  archegoniates,  where  it  is  the 
sporophyte  which  derives  its  nourishment  from  the 
tissue  of  the  large  green  gametophyte  in  which  it  is 
enclosed.  The  long  retention  of  the  germinating 
spore  within  the  sporangium  of  Selaginella  is  a 
further  step  in  the  direction  of  seed  formation. 

The  Origin  of  Seeds. — Much  light  has  been 
thrown  upon  the  history  of  the  seed  plants  by  the 
extensive  studies  of  the  Paleozoic  fossils,  which 


no  Plant  Life  and  Evolution 

have  been  made  within  recent  years.  It  is  now  evi- 
dent that  many  fossil  plants,  formerly  supposed  to 
be  true  ferns,  are  really  seed-bearing  plants  of  a 
very  primitive  kind;  and  it  is  practically  certain 
that  these  primitive  seed  plants  have  originated  more 
than  once  from  quite  distinct  types  of  ferns.  It  is 
clearly  proved  that  two  of  the  lowest  types  of  ex- 
isting seed  plants,  the  Cycads  and  the  curious 
Ginkgo,  or  maiden-hair  tree,  are  related  to  the  ferns. 
Some  of  the  fossil  club-mosses,  e.g.;  Lepidocarpon, 
were  also  seed-bearing  plants,  and  there  are  some 
pretty  strong  reasons  to  believe  that  these,  or  forms 
not  unlike  them,  may  have  been  the  progenitors  of 
another  characteristic  group  of  existing  seed-plants, 
the  Conifers. 

While  a  few  of  the  Paleozoic  horsetails  were  cer- 
tainly heterosporous,  heterospory  was  less  perfectly 
developed  in  this  group,  and  as  yet  there  is  no  evi- 
dence of  any  seed-bearing  plants  that  can  be  con- 
sidered to  be  allied  to  the  horsetails,  and  it  is  proba- 
ble that  this  phylum  never  advanced  beyond  the 
heterosporous  phase. 

The  Nature  of  the  Seed. — The  seed  is  a  further 
elaboration  of  the  megasporangium.  In  the  seed- 
plants  the  latter  generally  remains  attached  to  the 
sporophyte  until  the  spore,  which  is  permanently  re- 
tained within,  has  completed  its  development  and 
fertilization  has  been  effected.  The  embryo-plant, 
enveloped  in  the  double  covering  of  the  spore  mem- 
brane and  the  wall  of  the  sporangium,  is  very  ef- 


The  Origin  of  Land  Plants         lit 

fectively  protected  from  external  vicissitudes,  and, 
moreover,  during  its  development  can  draw  upon 
the  parent  plant  for  an  unlimited  supply  of  food. 
It  moreover  is  able  to  store  up  in  the  ripe  seed  the 
reserve  food  necessary  for  its  growth  during  the 
early  stages  of  germination.  The  advance  of  the 
resting  stage  of  the  plant,  from  the  simple  spore  in 
the  fern  to  the  embryo  within  the  seed  of  the  seed- 
plants,  gives  the  latter  a  great  advantage  in  the  cer- 
tainty and  rapidity  with  which  the  new  generation 
can  be  established. 

It  is  positively  known  that  the  development 
of  seeds  has  taken  place  quite  independently  in 
several  groups  of  pteridophytes,  and  this  makes 
it  likely  that  all  of  the  existing  seed-plants  are 
not  necessarily  descended  from  a  single  stock. 
There  is  no  question  of  the  origin  of  the  cycads 
and  Ginkgo  from  ferns  of  some  kind,  but  the 
evidence  of  the  origin  of  the  other  seed-plants  from 
the  same  source  is  not  so  convincing.  It  has  re- 
cently been  argued  by  Professor  Seward  that  Arau- 
caria,  one  of  the  conifers,  shows  strong  evidence  of 
a  relation  with  the  tree-like  club-mosses  of  the  Coal 
Measures,  and  there  is  much  to  be  said  in  favor  of 
the  derivation  of  the  conifers  from  quite  different 
forms  from  those  which  give  rise  to  the  cycads. 
All  of  the  so-called  Gymnosperms,  i.e.,  conifers, 
cycads,  etc.,  show  unmistakable  evidences  of  hav- 
ing originated  from  some  pteridophytic  ancestors, 
but  the  origin  of  the  higher  type- of  seed-plants,  the 


H2  Plant  Life  and  Evolution 

Angiosperms  or  ordinary  flowering  plants,  is  the 
subject  of  much  controversy. 

Alternation  of  Generations. — The  origin  of  the 
pteridophytes  is  a  matter  of  some  difference  of  opin- 
ion. The  weight  of  evidence  in  favor  of  the  deriva- 
tion of  the  leafy  sporophyte  of  these  plants  from 
the  sporogonium  of  some  form  allied  to  the  existing 
bryophytes  is  very  strong.  The  greater  elaboration 
of  the  sporogonium  in  such  forms  as  the  higher 
mosses  and  Anthoceros,  with  the  increasing  sub- 
ordination of  the  sporogenous  function,  shows  an 
unmistakable  tendency  towards  the  independence  of 
the  sporophyte  such  as  is  finally  reached  by  the 
ferns.  Between  the  latter  and  the  mosses  are 
marked  similarities  in  the  character  of  the  tissues, 
e.g.,  the  development  of  stomata,  the  green  assimi- 
lative tissue  of  the  assimilative  organ  of  the  sporo- 
phyte, and  finally  the  close  correspondence  in  the 
details  of  spore  division.  These,  together  with 
marked  similarities  in  the  structure  of  the  gameto- 
phyte  and  the  reproductive  organs,  make  it  exceed- 
ingly probable  that  the  alternation  of  generations  in 
the  pteridophytes  is  "  antithetic,"  as  it  is  in  the 
bryophytes ;  that  is,  the  sporophyte,  or  neutral  gen- 
eration, is  the  result  of  an  elaboration  of  the  strictly 
terrestrial  phase  of  the  plant's  life,  the  lineal  de- 
scendant of  the  unicellular  zygote  or  resting  spore 
of  some  green  alga,  developed  by  these  humble  water 
plants  for  enduring  periods  of  drought. 

A  good  many  cases  of  "  Apogamy,"  or  the  origin 


The  Origin  of  Land  Plants         113 

of  the  sporophyte  as  a  vegetative  bud  upon  the 
gametophyte,  have  been  recorded  for  various  ferns, 
and  it  has  also  been  found  that  the  gametophyte 
may  develop  directly  from  portions  of  the  sporo- 
phyte other  than  the  spores.  These  phenomena  have 
led  to  the  theory  that  there  is  no  essential  difference 
between  the  two  generations,  and  that  the  sporo- 
phyte is  not  to  be  considered  as  homologous  with 
the  sporogonium  of  the  bryophytes,  but  as  the 
direct  modification  of  some  gametophytic  structure. 

The  writer  has  pointed  out  more  than  once  that 
these  phenomena,  which  are  often  pathological,  may 
be  very  properly  compared  with  numerous  cases  of 
adventive  budding  or  regeneration  common  to  so 
many  of  the  higher  plants.  Of  course  there  is  an 
essential  structural  similarity  in  the  cells  of  the 
gametophyte  and  sporophyte,  each  of  which  is  nor- 
mally derived  from  special  cells  of  the  other,  egg 
or  spore  respectively.  It  is  not  surprising  then  that 
under  special  conditions,  in  view  of  the  great  power 
of  regeneration  exhibited  by  most  plant  tissues,  that 
the  phenomena  of  apogamy  and  apospory  should 
occur.  No  more  remarkable  than  the  production  of 
the  whole  plant  from  the  root  of  a  poplar,  or  from  a 
fragment  of  the  leaf  of  a  begonia,  where  it  has 
not  been  claimed  that  the  bud  is  in  one  case  homol- 
ogous with  the  root  and  in  the  other  with  the 
leaf. 

Antiquity  of  the  Pteridophytes. — It  is  clear,  from 
the  study  of  Paleozoic  fossils,  that  all  the  existing 


114  Plant  Life  and  Evolution 

classes  of  pteridophytes  are  of  great  antiquity,  and 
little  light  is  shed  upon  the  origin  of  these  types  by 
the  study  of  the  fossil  records,  as  the  bryophytic 
forms  from  which  the  pteridophytes  are  presumably 
descended  have  left  very  few  recognizable  traces, 
and  we  are  forced  to  fall  back  upon  the  study  of  the 
living  forms  for  clues  to  their  origin.  Of  the  liv- 
ing pteridophytes  several  types  have  been  assumed 
to  represent  the  nearest  approach  to  the  bryophytic 
type  of  sporophyte,  and  of  these  there  is  most  evi- 
dence in  favor  of  two — Ophioglossum  in  the  fern 
series,  and  Lycopodium  among  the  club-mosses.  It 
is  highly  probable  that  these  two  types,  which  are 
very  different,  represent  two  quite  independent 
classes  derived  from  different  ancestral  forms. 
These  ancestral  forms  may  not,  however,  have  dif- 
fered very  much  in  structure,  so  far  as  we  can 
judge,  and  are  most  nearly  represented  at  the  pres- 
ent time  by  Anthoceros.  As  we  have  seen,  the 
sporophyte  of  the  latter  is  relatively  long-lived, 
growing  for  many  months  and  developing  a  com- 
plete photosynthetic  apparatus,  and  like  the  pterido- 
phytes it  has  an  almost  independent  sporophyte,  ex- 
cept for  the  lack  of  external  organs,  leaves,  and 
roots.  In  Anthoceros  a  relatively  small  part  of  the 
sporogonium  is  devoted  to  spore  formation,  and 
there  is  a  certain  suggestion  of  the  sporangium  or 
spore-bearing  organs  of  the  pteridophytes. 

Bower's    Theory    of    Sterilization. — The    impor- 
tance of  the  progressive  sterilization  of  the  sporoge- 


The  Origin  of  Land  Plants         115 

nous  tissues  in  the  evolution  of  the  structures  of 
the  sporophyte  has  been  particularly  emphasized  by 
Professor  Bower,  who  has  recently  treated  this  sub- 
ject at  length.  (See  his  recent  book,  "  The  Origin 
of  a  Land  Flora.") 

Of  the  living  ferns  there  is  no  question  that 
Ophioglossum  approaches  more  nearly  than  any 
other  the  hypothetical  type  suggested  by  comparison 
with  Anthoceros.  In  this  fern  the  spore-bearing 
spike  (Fig.  12,  B)  shows  very  poorly  differentiated 
sporangia,  the  whole  being  comparable  with  some 
Anthoceros  type  in  which  the  segregation  of  the 
spore-masses  was  more  complete  than  in  the  exist- 
ing species. 

The  club-mosses  bear  the  sporangia  singly,  each 
sporangium  being  subtended  by  a  leaf,  and  these 
"  sporophylls "  together  often  form  a  cone  or 
"  strobilus."  In  the  ferns  the  leaves  are  large  and 
the  leaves  bear,  as  a  rule,  very  numerous  sporangia. 
Bower  thinks  that  the  ferns  may  also  be  considered 
to  represent  a  modification  of  the  strobiloid  type, 
but  there  are  strong  objections  to  be  brought  against 
this  view.  The  cone  in  the  horsetails  is  also  of  a 
very  different  type  from  that  of  the  club-mosses, 
and  probably  is  a  quite  independent  development.  It 
seems  to  the  writer  that  the  assumption  of  an  en- 
tirely separate  origin  for  the  type  of  sporophyte 
found  in  the  club-mosses  and  ferns  is  indicated  by 
the  data  now  available. 

Seed-bearing  Ferns. — The  great  importance  of 


n6  Plant  Life  and  Evolution 

the  pteridophytes  in  the  Paleozoic  flora  is  well 
known.  From  the  Devonian,  where  the  first  fern- 
like  remains  are  met  with,  they  increase  in  impor- 
tance, culminating  in  the  Carboniferous.  The  rich- 
ness of  the  Coal-flora  in  pteridophytes  is  sufficiently 
familiar.  It  is  now  known  that  many  of  the  sup- 
posed Paleozoic  ferns  were  really  seed-bearing 
plants  which  have  very  appropriately  been  named 
"  Pteridosperms  " — seed-ferns — and  some  enthusi- 
astic students  of  these  plants  have  gone  so  far  as 
to  doubt  the  presence  of  any  true  ferns  during  the 
Paleozoic,  a  view  which  it  is  hardly  necessary  to 
say  is  hardly  likely  to  prove  correct,  unless  we  sup- 
pose that  these  seed-ferns  originated  spontaneously 
and  had  no  ancestors. 

Distribution  of  Living  Pteridophytes. — It  is  usu- 
ally taken  for  granted  that  the  pteridophytes  of  the 
present  day  are  mere  remnants  of  the  rich  Paleozoic 
flora;  but  a  study  of  the  distribution  of  the  existing 
ferns  shows  that  this  is  not  the  case.  It  is  true 
that  the  living  horsetails  are  very  degenerate  de- 
scendants of  their  Paleozoic  ancestors,  and  the  same 
may  be  said  to  a  lesser  degree  of  the  club-mosses. 
The  case  of  the  true  ferns,  however,  is  quite  dif- 
ferent. The  fern  types  characteristic  of  the  earlier 
geological  epochs  have  largely  disappeared,  although 
there  are  some  ferns,  especially  in  the  tropics,  which 
have  changed  very  little  from  their  ancient  fore- 
bears. These  ancient  types  have  been  largely  re- 
placed by  ferns  which  are  better  adapted  to  modern 


The  Origin  of  Land  Plants         117 

conditions.  These  new  types  of  ferns  have  proved 
themselves  to  be  much  more  plastic  than  the  other 
pteridophytes,  and  many  types  have  arisen  which 
are  extremely  well  adapted  to  existing  condi- 
tions. 

In  some  especially  favorable  regions,  such  as  the 
higher  mountains  of  Jamaica,  and  in  New  Zealand, 
the  number  and  variety  of  the  ferns  is  extraordi- 
nary, and  they  are  perhaps  the  most  numerous  and 
conspicuous  plants  that  one  encounters.  From  the 
tiny  filmy  ferns,  sometimes  less  than  an  inch  in 
height,  to  the  majestic  tree  ferns  raising  their  mag- 
nificent crowns  of  fronds  thirty  or  forty  feet  above 
the  ground,  every  available  spot  is  occupied  by  a 
bewildering  variety  of  these  beautiful  plants. 
Moisture-loving  plants  as  they  are,  one  finds  that 
they  become  scarcer  in  the  drier  parts  of  the  world, 
but  many  species  have  become  adapted  to  dry 
regions.  For  instance,  there  are  a  number  of  ferns 
found  in  the  coast  regions  of  California,  where  for 
months  during  the  long  rainless  summer  they  be- 
come completely  dried  up,  and  apparently  lifeless, 
but  promptly  revive  with  the  advent  of  the  first 
autumn  rains.  In  the  moister  and  warmer  regions 
many  ferns  become  epiphytes  and  grow  upon  the 
trunks  and  branches  of  trees.  These  epiphytic 
ferns  are  among  the  most  beautiful  growths  that 
one  encounters  in  the  tropics.  A  few  species  of 
ferns  are  also  aquatic  in  habit,  but  the  number  of 
these  water  ferns  is  small. 


n8  Plant  Life  and  Evolution 

Persistence  of  Ancient  Types. — As  is  so  fre- 
quently the  case,  the  most  specialized  of  these  an- 
cient types  have  disappeared  before  their  still  more 
perfect  descendants,  while  the  lower  and  less  spe- 
cialized forms  have  persisted  or  have  left  de- 
scendants which  have  been  able  to  occupy  a  place 
to  which  more  highly  specialized  types  are  not  so 
well  adapted.  Thus  the  tree-like  pteridophytes  of 
the  Paleozoic  have  given  way  to  the  more  perfect 
modern  types  of  trees,  the  tree-ferns  alone  at  the 
present  day  reminding  us  of  their  past  glories.  But 
the  smaller  ferns  and  club-mosses  have  been  able  to 
compete  very  successfully  with  the  humbler  flower- 
ing plants  covering  the  floor  of  the  forest,  or  drap- 
ing the  banks  and  hillsides  in  the  moister  parts  of 
the  world. 

The  fossil  record  bearing  on  the  history  of  the 
ferns  and  their  allies  is  remarkably  complete,  and 
we  know  from  a  study  of  the  fossil  forms  that  all 
of  the  most  important  of  the  living  types,  i.e.,  ferns, 
horsetails,  and  club-mosses,  were  clearly  differen- 
tiated during  the  Devonian,  and  possibly  even  ear- 
lier. Some  of  the  early  fossil  types  have  persisted 
with  comparatively  little  change  down  to  the  present 
time,  while  in  others  the  changes  have  become  very 
marked  and  the  earlier  types  have  been  largely  dis- 
placed by  their  modified  descendants,  some  of  which 
have  adapted  themselves  very  satisfactorily  to  exist- 
ing conditions  even  in  the  temperate  regions.  Some 
species,  like  the  field  horsetail  and  the  bracken  fern, 


The  Origin  of  Land  Plants         119 

are  very  hardy  and  persistent,  and  in  the  more  fa- 
vorable conditions  of  the  moist  tropics,  ferns  con- 
stitute an  important  feature  of  the  vegetation,  and 
some  of  the  modern  tree-ferns  probably  equal  in 
size  any  of  their  Paleozoic  prototypes. 


CHAPTER  V 

SEED-PLANTS 

THE  MODERN  PLANT  TYPE 

A3  the  primitive  land  plants  adapted  themselves 
more  and  more  perfectly  to  the  increasingly 
diverse  conditions  associated  with  their  new  en- 
vironment, the  evidences  of  their  aquatic  ancestry 
became  less  and  less  apparent,  and  finally  in  the 
highest  of  all  plant  types,  the  flowering  plants  or 
seed-plants,  all  indications  of  their  derivation  from 
aquatic  ancestors  have  quite  disappeared. 

Mosses  and  Ferns,  Transitional  Forms. — The 
mosses  and  ferns  illustrate  the  transitional  stages 
through  which  the  seed-plants,  or  as  these  are  often 
called,  the  "  Phanerogams,"  have  passed  in  the 
course  of  their  evolution  from  their  primitive 
aquatic  ancestors,  the  green  algae.  It  is  evident  that 
the  course  of  this  evolution  has  proceeded  along 
several  quite  different  lines.  In  the  mosses,  or 
bryophytes,  the  history  of  the  gametophyte,  or  sex- 
ual phase  of  the  plant's  development,  illustrates  the 
limitations  of  this  aquatic  organism  in  adjusting 
itself  to  the  radically  different  water  conditions  to 


Seed-Plants  121 

which  land  plants  are  subjected.  These  limitations 
are  probably  due  to  the  fact  that  the  gametophyte  of 
the  archegoniates  is  essentially  a  water  plant.  Even 
the  most  perfect  garnet ophytes,  such  as  are  found 
in  the  higher  liverworts  and  mosses,  owing  to  their 
failure  to  develop  an  adequate  root  system  and  ef- 
ficient mechanical  or  supporting  tissues,  are  unable 
to  attain  any  but  the  most  modest  dimensions. 
Moreover,  these  plants  are  essentially  amphibious,  as 
water  is  necessary  to  effect  fertilization. 

In  the  ferns  the  development  of  the  race  centers  in 
the  sporophyte  or  neutral  generation.  The  sporo- 
phyte,  being  the  product  of  the  fertilized  ovum,  is 
equivalent  to  the  zygote  or  sexually  developed  rest- 
ing spore  of  the  ancestral  green  algae  from  which  the 
mosses  and  ferns  are  descended.  As  the  zygote  of 
these  algae  is  usually  adapted  to  survive  drought,  we 
may  say  that  the  sporophyte  has  never  been  an 
aquatic  structure,  but  from  its  earliest  beginning  is 
an  organism  fitted  for  terrestrial  existence.  It  evi- 
dently possesses  a  potentiality  for  development  on 
land  that  is  not  shared  by  the  essentially  aquatic 
gametophyte.  It  might  be  said  that  nature,  having 
in  the  mosses  exhausted  her  resources  in  the  en- 
deavor to  transform  the  aquatic  gametophyte  into 
a  successful  land  plant,  turned  to  the  spore-bearing 
generation  as  a  more  promising  subject  for  experi- 
mentation. In  the  ferns  there  is  encountered,  then, 
for  the  first  time,  a  sporophyte  which  possesses  true 
roots  having  sufficient  capacity  for  water  absorption 


122  Plant  Life  and  Evolution 

to  enable  it  to  supply  the  water  necessary  for  the 
further  development  of  the  sporophyte,  which  now 
becomes  a  perfectly  developed  land  plant,  with  stem, 
roots,  and  leaves  and  elaborately  developed  tissues. 
With  the  elaboration  of  this  sporophyte,  or  ter- 
restrial phase  of  the  plant's  life,  there  has  been  a 
gradual  reduction  of  the  aquatic  phase,  and  the 
gametophyte  becomes  more  and  more  insignificant, 
culminating  in  the  condition  met  with  in  the  hetero- 
sporous  pteridophytes,  in  which  the  sex  of  the  fu- 
ture gametophyte  is  already  indicated  by  the  char- 
acter of  the  spore.  This  tendency  to  heterospory  is 
shown  clearly  in  several  quite  independent  lines,  and 
just  as  the  different  types  of  sporophytes,  i.e.,  ferns, 
horsetails,  and  so  on,  probably  have  arisen  inde- 
pendently, so  heterospory  also  developed  in  various 
quite  different  lines.  In  some  of  these,  e.g.,  the 
water  ferns,  no  further  advance  seems  to  have  been 
made;  but  in  other  groups  a  further  development  of 
heterospory  resulted  in  the  formation  of  seeds,  the 
distinguishing  mark  of  the  highest  plants.  As  was 
pointed  out  in  the  last  chapter,  the  seed  is  not  a 
new  organ,  but  is  merely  an  elaboration  of  one  which 
already  existed,  the  megasporangium,  or  the 
sporangium  in  which  the  large  spores  or  mega- 
spores  are  developed,  and  from  the  latter  the  female 
gametophyte  is  produced.  There  is  abundant  evi- 
dence from  a  study  of  the  Paleozoic  pteridophytes 
that  seeds  developed  in  several  widely  separate 
groups,  and  this,  together  with  the  structure  of  the 


Seed-Plants  123 

living  seed-plants,  makes  it  pretty  certain  that  the 
existing  seed-bearing  plants  have  not  all  arisen  from 
the  same  stock. 

Selaginella. — Of  the  existing  pteridophytes  one 
genus  of  club-bosses  shows  a  remarkably  close  ap- 
proach to  the  seed-bearing  condition,  and  illustrates 
very  beautifully  the  intermediate  stage  between  the 
typical  pteridophytes  and  the  lowest  seed-bearing 
plants.  In  these  club-mosses  (Fig.  14,  A)  we  have 
seen  that  the  germination  of  the  megaspore  is  al- 
most entirely  completed  while  the  spores  are  still 
contained  within  the  sporangium,  and  the  growing 
gametophyte  is  nourished  by  food  substances  de- 
rived directly  from  the  cells  of  the  sporophyte,  and 
not  from  materials  stored  within  the  spore  itself,  as 
is  the  case  in  the  other  pteridophytes.  The  final 
stages,  however,  including  fertilization,  are  com- 
pleted after  the  spores  are  set  free,  and  as  in  the 
lower  pteridophytes,  water  is  necessary  to  convey 
the  sperms  to  the  open  archegonium. 

If  we  examine  the  "  flowers  "  of  one  of  the  lower 
seed-plants,  such  as  a  fir  or  pine  (Fig.  15,  B),  we 
shall  find  that  they  are  composed  of  closely  set 
scale-like  leaves  arranged  in  a  cone  which  is 
very  much  like  that  of  the  club-mosses.  These 
cones  are  of  two  kinds,  one  bearing  megasporangia 
like  those  of  Selaginella,  and  usually  denominated 
"  Ovules,"  the  other  bearing  the  microsporangia 
or  "  Pollen-sacs."  In  the  pine  there  are  two  ovules 
borne  upon  each  scale  of  the  young  cone,  and 


I24 


Plant  Life  and  Evolution 


FIG.  14 

Comparison  of  the  gametophytes  of  heterosporous  Pteri- 
dophytes  and  Gymnosperms. 

A — Germinating  megaspore  of  Selaginella,  showing  the  en- 
closed female  gametophyte,  $,  protruding  from  the  ruptured 
spore  apex. 

B — Section  of  a  microspore  of  Isoetes,  showing  the  very 
much  reduced  male  gametophyte  within,  v,  vegetative  cell  of 
gametophyte ;  two  sperms  can  be  seen  in  the  section. 

C — Section  of  megasporangium  (ovule)  of  a  pine.  A  single 
megaspore  is  present,  within  which  is  the  female  gametophyte, 
g,  with  the  archegonia,  ar.  Two  microspores  (pollen-spores) 
are  present  in  the  chamber  at  the  apex  of  the  ovule.  These 
have  sent  down  the  pollen  tubes. 

D — Section  of  the  ripe  megasporangium  (seed).  Within  the 
hard  shell  is  the  gametophyte,  g,  enclosing  the  young  sporo- 
phyte,  em,  derived  from  the  egg. 

E — Microspore  (pollen-spore)  of  Cycas;  from  the  anther- 
idial  cell,  an,  two  large  ciliated  sperms  are  developed. 

a    study    of    their    development    shows  that    their 
essential  structure  is  very  much  like  that  of  the 


Seed-Plants  125 

megasporangium  of  Selaginella.  Usually  but  one 
spore  matures,  but  this  is  very  large,  and  within  it 
arises  the  gametophyte  very  much  like  that  in  the 
megaspore  of  Selaginella,  and,  like  it,  producing  a 
number  of  archegonia.  The  megaspore,  however, 
is  never  set  free,  but  remains  permanently  within 
the  ovule,  and  this  necessitates  a  quite  different 
method  of  fertilization  (Fig.  14,  C). 

The  microsporangia  of  the  pine  are  also  in  pairs, 
but  are  upon  the  lower  side  of  the  sporophyll  or 
scale  which  bears  them.  The  development  of  the 
microsporangium  follows  very  closely  that  of  Se- 
laginella, and  the  spores  are  formed  in  groups  of 
four,  as  in  all  the  archegoniates.  These  micro- 
spores,  or  "pollen-spores"  (Fig.  14,  E),  give  rise 
to  a  rudimentary  male  gametophyte  with  two 
sperms,  which  are,  however,  destitute  of  cilia. 
When  the  ripe  pollen-spores  fall  upon  the  apex  of 
the  ovule  they  germinate,  sending  out  a  slender  tube 
which  pushes  its  way  through  the  tissues  overlying 
the  apex  of  the  megaspore,  and  the  two  sperms,  or 
generative  nuclei,  pass  into  the  pollen-tube  and  are 
thus  conveyed  to  the  archegonium.  The  develop- 
ment of  the  pollen-tube  does  away  with  the  neces- 
sity of  water  for  effecting  fertilization,  and  the  last 
evidence  of  the  aquatic  origin  of  these  plants  dis- 
appears. 

Motile  Sperms  in  Seed-plants. — One  of  the  most 
important  discoveries  of  recent  years  is  the  fact  that 
in  a  number  of  the  lowest  seed-plants  fertilization 


126  Plant  Life  and  Evolution 

is  still  effected  by  large  motile  sperms  very  much 
like  those  of  the  ferns.  The  only  plants  in  which 
this  has  been  found  are  the  fern-like  cycads,  in- 
cluding the  so-called  "  sago  palm  "  of  the  green- 
house, and  the  curious  Ginkgo,  or  maiden-hair  tree, 
which  is  not  uncommon  as  an  ornamental  tree. 
Both  of  these  types  had  long  been  recognized  as 
being  very  ancient  ones  and  as  having  very  close 
resemblances  to  the  ferns,  and  the  discovery  that 
they  both  develop  these  motile  sperms  practically 
makes  this  relationship  certain.  In  both  the  cycads 
and  Ginkgo,  the  female  gametophyte  is  not  essen- 
tially different  in  its  structure  from  that  of  the  pine 
or  Selaginella,  and  the  pollen  grain,  after  it  has 
fallen  upon  the  ovule,  also  develops  a  pollen-tube 
as  it  does  in  the  pine.  This  pollen-tube,  however, 
becomes  greatly  distended  by  an  accumulation  of 
water  and  finally  bursts,  discharging  the  two  enor- 
mous sperms,  together  with  the  water,  into  the 
chamber  which  lies  above  the  archegonia.  So  we 
see,  even  among  the  seed-plants,  there  may  still 
be  this  same  aquatic  type  of  fertilization  that  ob- 
tains in  the  whole  of  the  archegoniate  series  from 
which  these  plants  have  sprung.  In  a  Cuban  cycad, 
which  has  recently  been  described  by  Caldwell,  there 
may  be  as  many  as  sixteen  sperms  developed  in 
one  pollen-tube. 

In  the  pine  and  other  similar  types,  the  pollen- 
spore,  in  which  the  germination  is  well  advanced  at 
the  time  the  spore  is  shed,  shows  a  division  into 


ap 


127 


FIG.  15 

A  —  Megasporophyll  of  Cycas,  with  six  megasporangia,  or 
ovules,  o. 

B  —  Two  microsporangial  cones  of  a  spruce. 

C  —  Two  views  of  a  sporophyll  showing  the  two  sporangia, 
or  pollen-  sacs,  sp,  upon  its  lower  face. 

D  —  Two  cones  of  a  club-moss,  Lycopodium,  showing  a 
marked  superficial  resemblance  to  the  cones  of  the  spruce. 
The  sporophyll,  E,  has  a  single  sporangium  upon  its  upper 
side. 


several  vegetative  cells  and  an  antheridial  cell.    The 
germinating  spore  sends  out  its  pollen-tube,  which 


ia8  Plant  Life  and  Evolution 

penetrates  the  tissue  overlying  the  embryo-sac  very 
much  as  the  fungus  bores  its  way  through  the  tissues 
of  its  host.  In  many  of  the  conifers  there  is  an 
interim  of  nearly  a  year  between  the  time  of  pol- 
lination and  the  penetration  of  the  pollen-tube  into 
the  archegonium,  which  is  formed  long  after  the 
pollen  first  falls  upon  the  ovule. 

Fertilization. — With  the  discharge  of  the  sperm 
nucleus  into  the  egg,  and  its  fusion  with  the  egg- 
nucleus,  fertilization  is  consummated,  and  the  egg 
develops  into  the  embryo-sporophyte,  which  at  once 
begins  to  grow  until  the  young  organs — stem,  roots, 
and  leaves — are  well  advanced.  As  is  the  case  in 
Selaginella,  only  a  portion  of  the  egg  develops  into 
the  embryo  proper,  a  greater  or  less  amount  going 
to  form  a  peculiar  organ  known  as  the  suspensor, 
which  pushes  the  developing  embryo  into  the  mass 
of  gametophytic  tissue,  the  "  endosperm,"  whose 
cells  are  filled  with  starch,  oil,  and  albuminous  re- 
serve food  for  the  needs  of  the  young  sporophyte 
when  the  seed  germinates  (Fig.  14,  D).  The  wall 
of  the  sporangium,  together  with  the  accessory  en- 
velopes, or  integuments,  which  are  found  in  nearly 
all  seed-plants,  hardens  and  forms  the  characteristic 
shell,  or  testa,  upon  the  outside  of  the  seed.  It  is 
thus  clear  that  the  seed  is  not  a  new  formation, 
peculiar  to  the  seed-plants,  but  is  a  more  or  less 
perfectly  changed  megasporangium,  within  which 
is  contained  the  megaspore  with  its  enclosed  game- 
tophyte,  in  which  in  turn  is  embedded  the  embryo- 


Seed-Plants  129 

sporophyte,  so  that  the  ripe  seed  comprises  struc- 
tures belonging  to  three  generations. 

Advantages  of  the  Seed  Habit. — The  advantages 
of  the  seed  habit  are  apparent,  and  it  is  evident  that 
this  has  resulted  in  a  type  of  plant  peculiarly  adapted 
to  life  on  land.  This  is  shown  by  the  extraordinary 
development  of  seed-bearing  plants  at  the  present 
time.  Among  the  pteridophytes,  except  in  Selagi- 
nella,  which  is  probably  the  highest  genus  of  living 
pteridophytes,  the  developing  gametophyte  is  ex- 
posed to  the  vicissitudes  of  an  uncertain  water  sup- 
ply, free  water  being  essential  to  its  development 
and  for  fertilization.  In  the  seed-plants  the  gameto- 
phyte is  largely  protected  during  its  development, 
receiving  its  water  supply  indirectly  through  the 
tissues  of  the  sporophyte,  and  water  is  no  longer 
necessary  for  the  fertilization  of  the  ovum,  owing 
to  the  formation  of  the  pollen-tube.  Moreover,  the 
young  sporophyte  is  very  perfectly  protected  during 
its  early  development,  and  before  the  seed  ripens  it 
reaches  a  condition  where  it  is  ready  quickly  to  as- 
sume an  independent  condition. 

The  food  stored  up  in  the  seed  during  the 
process  of  ripening,  provides  an  ample  supply 
of  food  for  the  young  sporophyte  during  the 
early  stages  of  germination.  These  conditions 
give  the  seed-plants  a  tremendous  advantage  over 
most  pteridophytes,  except  under  very  special 
conditions,  either  where  the  latter  have  developed 
extraordinary  power  of  vegetative  reproduction, 


130  Plant  Life  and  Evolution 

as,  for  example,  the  extensively  spreading  root- 
stocks  of  the  horsetails  or  the  bracken  fern,  and 
in  some  of  the  club-mosses;  or  where  there  is  a 
constant  supply  of  moisture,  as  in  many  tropical 
mountain  regions  or  countries  like  New  Zealand. 
In  such  favored  regions  ferns  may  often  constitute 
an  important  factor  of  the  flora  and  hold  their 
own  very  successfully  with  the  seed-plants. 

The  First  Seed-plants. — The  lower  types  of  seed- 
plants  are  mostly  trees  or  shrubs  whose  resistant 
tissues  have  in  many  cases  been  preserved  in  a  fossil 
state  in  an  astonishingly  perfect  manner,  and  con- 
sequently the  geological  record  is  especially  satis- 
factory in  regard  to  these  important  forms.  A  study 
of  these  fossils  shows  that  the  seed  habit  arose  at 
a  very  early  period.  The  earliest  seed-bearing  plants 
were  very  different  from  any  living  types,  and  they 
have  been  separated  as  a  separate  order,  known  as 
the  Cordaitales.  The  affinities  of  the  latter  with 
other  plants  are  extremely  doubtful,  and  it  is  a 
question  whether  they  are  related  at  all  to  any  ex- 
isting species.  The  Cordaitales  are  found  as  far 
back  as  the  Devonian,  and  possibly  even  earlier,  and 
were  especially  abundant  during  the  Carboniferous. 
They  became  quite  extinct  before  the  end  of  the 
Paleozoic. 

Origin  of  Seed-plants. — The  fern-like  plants, 
which  are  first  certainly  evident  in  the  Devonian, 
where  many  forms  flourished,  like  all  of  the  types 
of  pteridophytes  had  an  extraordinary  develop- 


Seed-Plants  131 

ment  during  the  Carboniferous.  Recent  studies  of 
these  Carboniferous  "  ferns  "  show  that  many  of 
them  were  really  intermediate  in  character  between 
the  true  ferns  and  the  cycads,  and  many  of  them 
produced  true  seeds,  hence  the  very  proper  name  of 
Pteridosperms,  or  seed- ferns,  applied  to  these.  It 
is  highly  probable  that  from  some  of  the  pterido- 
sperms  the  cycads  are  directly  descended,  and  per- 
haps also  the  curious  Ginkgo,  which  has  been  re- 
ferred to,  whose  sole  representative  now  flourishes 
in  the  temple  gardens  of  China  and  Japan,  and 
occasionally  is  seen  in  our  parks  and  gardens.  The 
great  variety  of  these  seed-bearing  ferns  indicates 
that  the  seed  habit  was  developed  in  more  than  one 
line  of  ferns,  just  as  in  the  living  ferns,  the  two 
heterosporous  families,  the  Marsiliaceae  and  Sal- 
viniacese,  are  of  obviously  independent  origin. 

Among  the  fossil  club-mosses  of  the  Carbonifer- 
ous, there  are  also  unmistakable  evidences  of  seed- 
bearing  genera,  such  as  Lepidocarpon,  which  pre- 
sumably was  related  to  the  great  tree-like  Lepido- 
dendrons.  In  many  ways  these  seed-bearing  club- 
mosses  suggest  the  peculiar  conifers  of  the  South- 
ern Hemisphere,  Araucaria  and  Agathis,  and  it  is 
not  impossible  that  the  prevailing  modern  Gymno- 
sperms,  the  conifers,  are  the  descendants  of  some 
of  the  tree-like  Paleozoic  club-mosses.  It  must 
be  noted,  however,  that  this  view  is  strongly  op- 
posed by  some  eminent  students  of  the  Paleozoic 
fossils. 


132  Plant  Life  and  Evolution 

The  curious  horsetails,  or  scouring  rushes,  be- 
longing to  the  genus  Equisetum,  are  the  sole 
survivors  of  a  class,  which  in  the  early  geo- 
logical formations  was  represented  by  a  great 
variety  of  forms,  some  of  which  attained  tree-like 
dimensions.  Some  of  the  fossil  species  were  hetero- 
sporous,  but  there  is  no  evidence  that  any  of  them 
advanced  far  enough  to  develop  seeds,  and  so  far 
as  we  know  the  seed  habit  was  never  attained  by 
members  of  this  class  of  pteridophytes. 

Gymnosperms  and  Angiosperms. — The  seed- 
plants,  as  they  now  exist,  are  commonly  divided  into 
two  very  unequal  classes,  the  Gymnosperms  and  the 
Angiosperms.  The  gymnosperms,  which  include 
the  cone-bearing  evergreens,  are  the  older  type,  and 
in  these  the  gametophyte  shows  obvious  resem- 
blances to  that  of  their  pteridophytic  forebears,  and 
the  homologies  are  sufficiently  evident.  The  angio- 
sperms,  on  the  other  hand,  the  ordinary  flowering 
plants,  which  comprise  an  overwhelming  majority 
of  existing  seed-plants,  show  much  less  evidence  of 
their  origin  from  lower  forms,  and  at  present  it  is 
an  open  question  whether  or  not  they  are  at  all  re- 
lated to  any  of  the  existing  gymnosperms.  It  is, 
moreover,  very  unlikely  that  all  of  the  existing 
gymnosperms  have  had  a  common  origin.  The  two 
lowest  types,  the  cycads  and  Ginkgo,  are,  with  very 
little  question,  descended  from  fern-like  ancestors, 
presumably  through  some  types  of  the  seed-bearing 
ferns  of  the  Carboniferous.  This  is  certainly  true 


Seed-Plants  133 

of  the  cycads,  which  both  in  form  and  internal 
structure  reveal  unmistakably  their  fern  ancestry. 
Moreover,  the  discovery  of  fern-like  sperms  in  these, 
and  the  actual  development  of  green  tissue  in 
the  gametophyte  under  certain  conditions,  show 
that  they  are  very  much  less  reduced  in  these  respects 
than  some  of  the  living  heterosporous  pteridophytes, 
and  indicate  still  further  the  extremely  primitive 
nature  of  these  low  seed-bearing  plants. 

In  the  conifers,  the  adaptation  to  terrestrial  con- 
ditions is  complete,  and  all  trace  of  their  aquatic 
ancestry  is  finally  lost.  These  are  the  predominant 
type  of  gymnosperms  at  the  present  day,  and  are 
not  certainly  met  with  until  the  last  of  the  Paleozoic 
formations,  where  in  the  form  of  some  Permian 
species  there  is  evidence  of  the  beginning  of  the 
coniferous  series.  As  we  have  already  indicated, 
there  is  some  reason  to  suppose  that  these  conifers 
may  be  descendants  of  some  of  the  gigantic  club- 
mosses  of  the  Carboniferous,  but  some  eminent  au- 
thorities believe  that  the  conifers  also  are  descended 
from  fern-like  ancestors. 

Gnetales. — The  last  order  of  the  gymnosperms, 
the  Gnetales,  is  a  small  one  containing  three  very 
peculiar  genera,  with  a  small  number  of  species, 
evidently  not  at  all  closely  related  either  to  each 
other  or  to  the  other  gymnosperms.  Unfortunately 
these  plants  are  practically  unknown  in  a  fossil 
condition,  and  at  present  it  is  impossible  to  deter- 
mine their  exact  position  in  the  system. 


134  Plant  Life  and  Evolution 

The  Gymnosperms  Not  a  Homogeneous  Class. — 
From  the  above  statements  it  is  evident  that  the 
gymnosperms  do  not  constitute  a  homogeneous  as- 
semblage of  plants,  but  represent  a  more  or  less 
heterogeneous  collection  of  forms,  which  may  very 
well  represent  several  quite  unrelated  lines  of  de- 
scent. They  all  agree  in  having  seeds  of  a  primitive 
type,  usually  exposed  upon  open  leaves,  or  sporo- 
phylls;  whence  their  name  of  Gymnosperms,  or 
"  naked-seeded  "  plants.  They  are  evidently  less 
fitted  to  existing  conditions  than  their  rivals,  the 
angiosperms,  which  have  largely  superseded  them 
and  have  shown  a  far  greater  power  of  adaptation, 
clearly  indicated  by  their  enormously  greater  vari- 
ety of  species  and  individuals.  Probably  there  are 
not  more  than  five  hundred  living  species  of  gymno- 
sperms, \vhile  of  the  angiosperms  more  than  one 
hundred  thousand  already  have  been  described. 

THE  CYCADS 

During  the  Paleozoic,  especially  in  the  Car- 
boniferous, there  arose  a  great  assemblage  of  fern- 
like  plants,  showing  a  wide  range  of  structure, 
many  of  them  approaching  both  in  the  structure  of 
the  tissues  and  in  their  reproductive  parts  the  lower 
types  of  seed-bearing  plants.  Some  of  these 
pteridosperms,  or  seed-bearing  ferns,  were  evidently 
not  very  different  in  appearance  from  some  of  the 
living  ferns,  especially  those  belonging  to  a  small 


Seed-Plants  135 

order  of  mostly  tropical  ferns,  the  Marattiaceae, 
with  which  there  is  little  question  that  they  are 
directly  related.  Others  were  more  like  the  lowest 
of  the  existing  seed-plants,  the  Cycads,  and  were 
probably  the  direct  progenitors  of  the  latter.  The 
cycads  become  more  important  in  the  later  Paleozoic 
formations,  but  reach  their  maximum  development 
in  the  next  great  geological  epoch,  the  Mesozoic. 

The  predominance  of  cycads  in  the  early  Meso- 
zoic suggests  that  the  climate  of  that  period  very 
materially  differed  from  that  of  the  Carboniferous. 
The  rank  growth  of  pteridophytes  at  that  time 
must  have  been  conditioned  by  an  excess  of  mois- 
ture, and  a  probably  very  even  temperature.  The 
modern  cycads  are  for  the  most  part  plants  of  the 
sub-tropical  and  drier  tropical  regions,  where  they 
are  usually  subjected  to  more  or  less  extended  peri- 
ods of  drought.  It  may  be  that  increasing  dryness 
was  one  cause  of  the  tendency  to  seed-formation  in 
the  later  Paleozoic  time.  It  is  true  that  there  was 
one  group  of  seed-plants,  the  Cordaitales,  already 
well  developed,  which  became  extinct  before  the  end 
of  the  Paleozoic,  while,  as  we  know,  seeds  were  de- 
veloped in  various  of  the  fern-like  plants  and  in 
the  club-mosses.  But  there  is  some  evidence  that 
even  during  the  Paleozoic  there  were  fluctuations 
in  the  amount  of  moisture,  and  it  is  possible  that 
these  fluctuations  may  coincide  to  some  extent  with 
the  periods  of  seed  formation. 

The  cycads  have  now  given  place  largely  to  the 


136  Plant  Life  and  Evolution 

more  modern  types  of  seed-plants,  but  there  are 
still  some  seventy-five  species  of  these,  pretty  well 
distributed  over  the  warmer  parts  of  the  earth.  Of 
these,  the  genus  Cycas,  represented  by  the  common 
C.  revoluta,  the  "  Sago  Palm  "•  of  the  florist,  is  espe- 
cially interesting,  as  it  is  a  survivor  of  one  of  the 
earliest  genera  known,  and  has  come  down  probably 
from  the  early  Mesozoic  with  apparently  little 
change.  Cycas  is  in  habit  very  much  like  a  tree- 
fern.  The  upright  trunk  bears  at  its  summit  a 
crown  of  fern-like  leaves,  which  when  young  have 
the  leaflets  coiled  up  like  those  of  a  young  fern- 
leaf.  The  fertile  leaves,  or  sporophylls,  in  Cycas 
retain  the  fern-like  form  (Fig.  15,  A),  and  the 
enormous  ovules,  or  megasporangia,  are  borne  on 
the  margins  of  the  sporophylls,  and  later  develop 
into  great  seeds,  which  in  some  species  are  as  big  as 
a  hen's  egg. 

Although  the  gametophyte  is  well  advanced  in  the 
big  seeds,  its  final  development  and  fertilization 
take  place  after  the  seed  has  fallen  off  of  the 
plant.  The  gametophyte  has  also  been  known  in 
some  cases,  where  fertilization  was  not  effected,  to 
continue  its  growth  and  develop  a  green  mass  of 
tissue  like  the  gametophyte  of  the  lower  ferns,  indi- 
cating that  as  heterospory  developed  in  the  fern  an- 
cestors of  the  cycads,  the  reduction  of  the  gameto- 
phyte was  much  less  than  in  that  of  the  existing 
heterosporous  ferns.  This  great  development  of  the 
gametophyte,  together  with  the  presence  of  motile 


Seed-Plants  137 

sperms,  emphasizes  the  very  low  rank  of  these  primi- 
tive seed-plants. 

Cycadeoideae. — Either  from  the  cycads,  or  per- 
haps independently  from  some  of  the  Paleozoic 
pteridosperms,  there  arose  a  second  group  of 
cycad-like  plants,  which  also  culminated  in  the 
Mesozoic,  and  were  much  more  specialized  than 
any  of  the  true  cycads.  These  plants  have  been 
called  "  Cycadeoideae,"  and  the  most  important  col- 
lections of  these  have  been  made  from  the  Black 
Hills  region  of  Dakota  and  Wyoming.  From  a 
study  of  these  fossils  (for  details  see  Wieland: 
"American  Fossil  Cycads  "),  which  have  been  very 
well  preserved,  our  knowledge  of  the  structure  of 
these  remarkable  forms  is  very  complete.  In  some 
of  these  Cycadeoideae  the  "  flowers  "  have  been  very 
well  preserved,  and  the  arrangement  of  the  sporo- 
phylls,  which  are  borne  together  upon  the  same  cone, 
is  so  much  like  that  of  such  flowers  as  the  water- 
lily  or  magnolia,  that  some  students  have  actually 
claimed  that  these  Cycadeoideae  are  the  real  an- 
cestors of  the  higher  flowering  plants,  the  Angio- 
sperms.  It  must  be  remembered,  however,  that  the 
Cycadeoideae  are  gymnosperms,  that  is,  the  naked 
seeds  are  borne  free  upon  the  sporophylls  and  the 
"  stamens "  are  very  unlike  those  of  the  angio- 
sperms,  and  closely  resemble  the  leaves  of  the 
true  ferns.  Moreover,  it  may  be  assumed  that 
the  gametophyte  was  well  developed,  like  that  of 
the  cycads,  and  it  would  certainly  be  rash 


138  Plant  Life  and  Evolution 

to  assume  a  direct  connection  between  these  forms 
and  any  of  the  higher  plants,  until  we  have  a 
great  deal  more  evidence  upon  the  subject.  Never- 
theless, the  possibility  of  an  origin  of  the  higher 
plants  from  some  gymnospermous  forms,  allied  to 
the  cycads,  must  be  borne  in  mind  in  any  specula- 
tions as  to  the  origin  of  the  angiosperms. 

GIN  KGO  ALES 

The  curious  maiden-hair  tree,  or  Ginkgo,  which 
we  have  already  referred  to,  is  the  sole  survivor  of 
an  extremely  ancient  race  which  was  represented 
by  many  species  in  the  later  Paleozoic  and  early 
Mesozoic,  some  of  the  later  Paleozoic  species  having 
actually  been  referred  to  the  existing  genus  Ginkgo. 
In  the  temple  gardens  of  Japan  are  many  superb 
specimens  of  this  strange  tree.  These  are  sometimes 
of  great  size,  and  are  said  to  be  many  centuries  old. 
Unlike  the  cycads,  the  tree  is  extensively  branched, 
and  looks  not  unlike  a  poplar.  Its  curious  fan- 
shaped  leaves,  which  are  deciduous  show  ?,  forked 
venation  like  that  of  a  maiden-hair  fern ;  hence  the 
name,  "  maiden-hair  tree,"  sometimes  applied  to  it. 

The  large  seeds,  which  are  borne  at  the  end  of 
short  branches,  are  much  like  those  of  Cycas,  and  the 
structure  of  the  gametophyte  and  the  development 
of  large  motile  sperms  in  the  pollen-tube  are  strik- 
ingly similar.  It  seems  probable  that  there  is  a  real 
relationship  between  Ginkgo  and  the  cycads,  but  it 


Seed-Plants  139 

is  probably  a  remote  one.  There  does  not,  however, 
seem  to  be  any  strong  evidence  of  any  direct  rela- 
tion with  any  other  existing  plant,  although  for- 
merly Ginkgo  was  associated  with  the  conifers,  and 
there  are  some  undoubted  resemblances  between 
them,  such  as  the  character  of  the  wood  and  the 
seed.  For  a  long  time  it  was  supposed  that  the  tree 
was  extinct  in  a  wild  condition,  but  it  has  finally 
been  discovered  growing  wild  in  certain  parts  of 
western  China. 

CONIFERALES 

The  great  majority  of  the  living  gymnosperms 
are  Conifers,  the  ordinary  cone-bearing  ever- 
green trees,  which  in  certain  regions,  like  the  Pacific 
Slope  of  North  America,  are  the  dominant  forest 
trees.  There  is  much  difference  of  opinion  as  to 
the  origin  of  the  conifers,  but  we  believe  that  the 
weight  of  evidence  is  in  favor  of  their  derivation 
from  some  types  allied  to  the  tree-like  club-mosses 
of  the  Paleozoic.  The  great  club-mosses,  like 
Lepidodendron  and  Sigillaria,  although  they  were 
undoubtedly  different  in  many  ways,  nevertheless 
recall  in  certain  respects  the  modern  coniferous 
trees.  Like  the  latter,  they  developed  a  secondary 
growth  in  thickness,  and  the  cones  are  quite  sim- 
ilar. As  some  of  these  Paleozoic  club-mosses  are 
known  to  have  developed  seeds,  which  recall  those 
of  the  living  Araucaria,  the  derivation  of  the  latter 


140  Plant  Life  and  Evolution 

from  such  seed-bearing  lycopods  is  by  no  means 
improbable.  This  is,  however,  by  no  means  so  obvi- 
ous as  the  derivation  of  the  cycads  from  fern-like 
forms;  but  if  it  should  be  demonstrated,  of  course 
it  would  indicate  that  the  two  orders  of  gymno- 
sperms — conifers  and  cycads — are  absolutely  un- 
related. Some  students  of  the  fossil  gymnosperms 
consider  the  Cordaitales,  which  are  the  oldest  of 
all  known  seed-plants,  to  be  a  composite  type  with 
affinities  both  with  conifers  and  cycads;  but  the 
arguments  brought  forward  in  favor  of  this  theory 
are  not  entirely  convincing,  and  it  is  quite  as  likely 
that  there  is  no  direct  relationship  between  the  Cor- 
daitales and  any  living  gymnosperms. 

The  earliest  known  Conifers,  which  were  proba- 
bly allied  to  the  living  genus  Araucaria,  which  in- 
cludes the  Norfolk  Island  pine,  are  met  with  in  the 
later  Paleozoic,  from  which  time  they  increase  rap- 
idly in  number  and  importance,  until  by  the  end  of 
the  Mesozoic,  practically  all  of  the  existing  genera 
are  met  with.  With  the  advent  of  the  angiosperms, 
which  began  to  be  prominent  by  this  time,  the 
conifers  decline  in  importance  but  still  have  held 
their  own  pretty  well,  and  are  important  constitu- 
ents of  the  floras  of  many  parts  of  the  world;  but 
their  importance  is  due  rather  to  number  of  indi- 
viduals than  to  any  great  variety  of  species.  The 
conifers  have  shown  themselves  to  be  far  more 
adaptable  than  the  cycads,  which  at  present  very 
seldom  occur  anywhere  in  large  numbers,  while  the 


Seed-Plants  141 

conifers,  being  very  frequently  gregarious  in  habit, 
may  form  almost  perfectly  pure  forests  of  a  single 
species. 

The  most  obvious  difference  between  cycads  and 
conifers  is  the  different  relation  of  stem  and  leaves, 
the  same  that  distinguishes  the  ferns  and  club- 
mosses.  The  simple,  or  sparingly  branching  palm- 
like  trunk  of  the  cycads,  with  its  crown  of  fern-like 
fronds,  is  extremely  different  from  the  extensively 
branched  conifer,  with  its  scattered,  slender,  and 
often  needle-shaped  leaves.  Most  of  the  more  fa- 
miliar conifers,  like  the  pines  and  firs,  are  more  or 
less  markedly  xerophytic,  i.e.,  their  foliage  is 
adapted  to  check  excessive  transpiration.  This  may 
be  correlated  with  their  growth  in  dry  regions 
where  they  are  exposed  to  the  hot  sun,  or  it  may 
be  in  the  case  of  the  Northern  species  an  adaptation 
to  prevent  loss  of  water  during  the  winter,  a  con- 
dition which  the  Northern  angiospermous  trees  meet 
by  casting  their  leaves.  Species  growing  in  moister 
regions  have  softer  and  broader  leaves,  and  this  is 
noticeably  the  case  in  some  of  the  presumably  more 
primitive  types,  like  Araucaria  and  the  Kauri  pine 
of  New  Zealand,  which  have  relatively  broad  leaves 
and  are  decidedly  less  xerophytic  in  habit  than  the 
pines  and  spruces  of  the  North.  While  most  of  the 
conifers  are  evergreens,  a  few,  like  the  larch,  and 
the  bald  cypress  of  the  Gulf  region,  cast  their  leaves 
in  autumn,  behaving  thus  like  deciduous  angio- 
spermous trees. 


142  Plant  Life  and  Evolution 

Some  conifers  are  quite  adaptable,  certain  species 
growing  under  very  unfavorable  conditions,  like 
the  pines  and  firs  of  the  alpine  summits  of  the 
Sierras  and  Rocky  Mountains,  or  the  barren  slopes 
overlooking  the  desert.  A  few  species,  like  the 
Monterey  pine  and  cypress,  grow  where  they  are 
exposed  to  the  full  force  of  the  ocean  winds.  They 
reach  their  finest  development  on  the  western 
slopes  of  the  great  mountain  chains  bordering  Pa- 
cific North  America,  and  are  also  highly  developed 
in  the  Manchurian  and  Japanese  region  on  the  other 
side  of  the  Pacific.  The  conditions  of  the  Pacific 
slope  of  North  America  seem  to  be  better  fitted  for 
the  needs  of  the  coniferous  trees  than  those  of  any 
other  part  of  the  world.  A  very  temperate  and  uni- 
form climate,  with  abundant  moisture,  especially  to- 
wards the  north,  perhaps  represents  to  some  extent 
the  climatic  conditions  of  the  later  Mesozoic  and 
the  earlier  Tertiary,  when  the  ancestors  of  the  pres- 
ent coniferous  flora  flourished;  and  the  giants  of  the 
vegetable  kingdom  have  developed  in  these  Pacific 
forests.  The  mighty  Sequoias,  the  last  of  their  race, 
tower  above  all  the  other  trees  of  the  forest;  but 
giant  pines,  firs,  and  cedars,  which  accompany  them, 
are  unrivaled  in  size  except  by  the  Sequoias,  and 
make  up  a  forest  that  is  unequaled  in  all  the  world. 

That  some  of  these  conifers,  like  the  Sequoias  and 
wild  nutmegs  of  the  California  forest,  or  the  cy- 
presses of  the  southern  swamps,  were  once  wide- 
spread trees  is  plainly  shown  by  the  fossil  remains 


Seed-Plants  143 

in  many  regions  both  of  the  Old  and  New  World, 
where  it  would  be  quite  impossible  for  them  now  to 
exist.  Great  changes  must  have  taken  place  in  the 
climate  since  the  time  when  these  trees  were  com- 
mon over  much  of  the  Northern  Hemisphere,  where 
now  they  maintain  only  a  very  precarious  existence 
in  a  few  places  where  unusually  favorable  condi- 
tions have  permitted  them  to  survive. 

GNETALES 

Distributed  through  the  Tropics  of  both  the  Old 
and  New  World,  there  are  found  about  a  dozen 
species  of  trees  and  woody  climbers  belonging  to  the 
genus  Gnetum.  Some  of  these  are  lianas,  climbing 
to  the  tops  of  lofty  trees.  The  opposite,  oval, 
pointed  leaves  are  net-veined  like  those  of  the 
typical  dicotyledons,  and  the  flowers,  borne  in 
catkin-like  spikes,  may  be  compared  to  those  of  a 
poplar  or  willow.  The  plant,  however,  is  "  gym- 
nospermous  " ;  that  is,  the  seeds  are  not  contained 
in  an  ovary,  and  in  this  respect  they  agree  with  the 
conifers  and  cycads;  but  otherwise  Gnetum  has  lit- 
tle in  common  with  the  other  gymnosperms,  nor  is 
its  relationship  to  the  other  genera,  Ephedra  and 
Welwitschia,  which  are  associated  with  it  to  form 
the  order,  Gnetales,  at  all  certain.  Gnetum  is  some- 
times held  to  be  intermediate  between  gymnosperms 
and  angiosperms,  but  the  evidence  for  this  is  by 
no  means  decisive. 


144  Plant  Life  and  Evolution 

The  second  genus  of  Gnetales,  Ephedra,  with 
some  twenty  species,  is  represented  in  the  United 
States  by  two  or  three  species  of  the  arid  Southwest. 
In  the  Colorado  desert,  and  the  deserts  of  southern 
Arizona,  one  frequently  meets  with  these  straggling 
bushes,  whose  leafless,  gray-green,  jointed  stems,  re- 
mind one  of  the  horsetails.  While  these  bushes  are 
very  different  in  aspect  from  most  conifers,  the 
structure  of  the  seeds  is  quite  similar,  this  being 
especially  the  case  with  the  highly  developed 
gametophyte. 

The  third  genus  of  the  Gnetales,  Welwitschia,  has 
but  a  single  species,  confined  to  a  limited  desert 
region  in  Western  Africa.  This  extraordinary  plant 
has  a  short,  thick  trunk,  tapering  into  a  long  tap- 
root, so  that  it  looks  like  a  great  carrot  or  parsnip 
with  the  top  cut  off.  Growing  from  the  margin  of 
the  flattened  trunk-apex  are  two  immense  strap- 
shaped  leaves,  which  are  all  the  plant  has.  From 
the  margin  of  the  trunk  cones  of  flowers  are  also 
produced.  These  are  made  up  of  large  red  scales, 
which  are  very  conspicuous,  and,  it  is  said,  attract 
insects  so  that  the  plant  is  sometimes,  at  least,  cross- 
pollinated.  The  more  recent  studies  on  this  plant 
by  Professor  Pearson,  of  Cape  Town,  indicate  cer- 
tain resemblances  in  structure  to  Gnetum,  and  it 
is  not  impossible  that  the  two  genera  are  remotely 
related.  Ephedra  probably  is  not  related  to  either 
of  the  other  genera.  Practically  nothing  is  known 
of  the  geological  history  of  the  Gnetales,  but  their 


Seed-Plants  145 

present  distribution  indicates  that  they  are  probably 
pretty  old  types. 


RESUME 

Seed-plants  Not  All  of  Common  Origin. — That 
the  seed  habit  developed  a  number  of  times  in  quite 
unrelated  groups  of  pteridophytes  is  amply  shown 
by  the  fossil  remains  of  seed-bearing  plants  in  the 
Paleozoic.  Some  one,  or  perhaps  more  than  one,  of 
the  seed-bearing  ferns  were  probably  the  progen- 
itors of  the  existing  cycads,  and  some  of  the 
more  specialized  cycad-like  forms  of  the  Mesozoic 
formations.  It  is  also  pretty  certain  that  the  pe- 
culiar genus  Ginkgo  is  also  descended  from  some 
fern-like  Paleozoic  type.  The  cycads  of  the  pres- 
ent time  are  much  scattered,  and  seldom  occur  in 
numbers  to  make  them  important  constituents  of  a 
flora.  The  existing  types  are  descended  from  some 
of  the  less  specialized  fossil  ones. 

The  conifers  are  decidedly  the  prevailing  type  of 
gymnosperms  at  the  present  day.  Although  the 
number  of  known  species  is  less  than  four  hun- 
dred, they  are  nevertheless  very  important  factors  in 
the  existing  flora  of  many  parts  of  the  world,  as 
they  often  form  extensive  forests,  including  the 
largest  of  known  trees.  The  order  is  an  adaptable 
one,  and  conifers  grow  under  quite  different  condi- 
tions; but  there  are  no  real  aquatic  forms,  though 
some,  like  the  cypress  and  tamarack,  are  swamp 


146  Plant  Life  and  Evolution 

trees.  The  origin  of  the  conifers  is  uncertain,  but 
there  is  some  reason  to  assume  a  descent  from 
forms  allied  to  the  giant  club-mosses  of  the  Coal 
Measures.  The  affinities  of  the  other  order  of  gym- 
nosperms,  the  Gnetales,  are  not  at  all  clearly  estab- 
lished, nor  is  it  certain  that  the  three  known 
genera  are  related  among  themselves.  There  is 
some  ground  for  an  assumption  of  a  relationship 
between  some  of  these  and  the  lower  angiosperms ; 
but  the  origin  of  the  latter,  the  predominant  modern 
type  of  plants,  is  very  far  from  clear. 

The  Living  Seed-plants  Not  All  Related. — It  is, 
however,  extremely  likely  that  the  existing  seed- 
bearing  plants  do  not  form  a  homogeneous  class, 
but  more  properly  should  be  considered  as  a  class 
representing  several  quite  independent  develop- 
mental lines,  some  of  which  may  not  be  related  at 
all.  The  lower  types,  or  gymnosperms,  show  evi- 
dent relationship  with  the  pteridophytes,  but  the 
different  orders  may  very  well  have  been  derived 
from  quite  different  pteridophytic  stocks.  The 
origin  of  the  modern  type  of  flowering  plants,  the 
angiosperms,  is  exceedingly  obscure.  One  group 
of  seed-plants,  the  Cordaitales,  the  earliest  known 
type,  became  extinct  at  the  end  of  the  Paleozoic,  and 
probably  has  left  no  descendants. 


CHAPTER  VI 
THE  ANGIOSPERMS 

THE  evolution  of  the  seed  marks  the  final  step 
in  the  complete  adjustment  of  the  plant  or- 
ganism to  strictly  terrestrial  existence,  and  while 
seeds  undoubtedly  arose  independently  in  several 
widely  separate  classes,  most  of  these  primitive  seed- 
plants  have  disappeared  completely,  or  have  left  only 
a  few  descendants,  which  maintain  a  more  or  less 
precarious  existence  at  the  present  time.  One  line 
of  seed-plants,  however,  has  proved  itself  eminently 
adapted  to  modern  conditions,  and  constitutes  an 
overwhelming  majority  of  living  plants.  These 
prevailing  flowering  plants  of  the  present  time  are 
known  as  Angiosperms.  In  the  angiosperms  the 
plant  organism  reaches  its  most  perfect  expression, 
and  they  now  dominate  the  land  floras  of  all  parts 
of  the  world.  Plastic  to  a  degree  unequaled  by  any 
other  plants,  they  have  succeeded  in  adapting  them- 
selves to  the  most  diverse  conditions.  In  the  burn- 
ing deserts  of  the  Tropics,  at  the  utmost  limits  of 
vegetation  in  the  polar  zones,  and  on  mountain  sum- 
mits, angiosperms  have  made  themselves  at  home. 
Some  have  even  invaded  the  sea,  while  still  others 
are  inhabitants  of  fresh- water  marshes  or  are  sub- 
147 


148  Plant  Life  and  Evolution 

mersed  in  lakes  or  rivers.  Like  the  fungi,  many 
angiosperms  are  parasites  or  saprophytes  in  their 
habits,  and  like  the  fungi  these  may  be  quite  desti- 
tute of  chlorophyll,  and  must,  therefore,  depend 
upon  other  organisms  for  their  supply  of  organic 
food. 

Adaptability  of  Angiosperms. — Perhaps  in  no 
way  is  the  adaptability  of  the  angiosperms  better 
shown  than  in  their  relation  to  the  animal  world. 
Serving  as  plants  do  for  the  food  of  a  vast  number 
of  animal  forms,  there  have  been  evolved,  in  the 
course  of  the  development  of  both  the  animal  and 
plant  kingdoms,  numberless  cases  of  special  adapta- 
tions of  which  the  plants  have  taken  advantage. 
This  is  seen  perhaps  most  perfectly  in  the  evolution 
of  flowers,  whose  peculiarities  are  very  generally 
associated  with  cross  pollination  through  the  agency 
of  insects  or  birds.  The  peculiar  modifications  of 
the  latter,  for  instance,  the  mouth  parts  of  bees  or 
butterflies,  or  the  beak  of  the  humming  birds  or 
honey  suckers,  are  clearly  correlated  with  floral 
structures.  Many  types  of  fruits  also  are  associated 
with  modifications  in  animal  structures.  The  teeth 
of  many  mammals,  and  the  beaks  of  certain  birds, 
are  undoubtedly  adaptations  for  feeding  upon  cer- 
tain types  of  fruits,  and  the  importance  of  animals, 
especially  birds,  in  the  distribution  of  the  seeds  of 
many  plants  is  sufficiently  well  known. 

Although  it  is  almost  certain  that  all  of  the  angio- 
sperms have  arisen  from  a  common  stock,  the  range 


The  Angiosperms  149 

of  their  structures  is  extraordinarily  great,  illus- 
trating again  their  remarkable  powers  of  adaptation. 
Some  of  them  are  tiny,  almost  microscopic  water 
plants  of  extraordinarily  simple  structure;  others 
are  humble  weeds,  completing  their  whole  life  in 
the  course  of  a  few  weeks,  while  still  others  are 
giant  trees,  living  hundreds  of  years.  With  all  this 
extraordinary  variation  in  size,  form,  and  habit,  the 
fundamental  structure  of  the  flower,  the  character- 
istic mark  of  the  angiosperms,  is  very  uniform, 
but  is  so  different  from  that  of  any  gymnosperms 
as  to  make  the  origin  of  the  angiosperms  a  matter 
of  great  uncertainty. 

Angiosperms  Absent  from  the  Early  Rocks 

While  the  ferns  and  gymnosperms  have  left  abun- 
dant and  well-preserved  fossil  remains  whose  nature 
is  unmistakable,  of  the  angiosperms,  except  in  the 
later  geological  formations,  only  scanty  traces  are 
discernible,  and  these  are  often  very  uncertain  in 
their  nature.  While  a  good  many  fragments  of 
leaves  and  stems  from  Paleozoic  and  early  Meso- 
zoic  rocks  have  been  assigned  to  angiosperms,  these 
fragments  in  most  cases  are  very  poorly  preserved, 
and  their  real  nature  is,  to  say  the  least,  problem- 
atical. The  tendency  among  recent  students  of  these 
fossils  is  to  relegate  them  either  to  pteridophytes  or 
gymnosperms. 

It  is  not  until  the  later  Mesozoic  formations 
are  reached  that  unmistakable  remains  of  angio- 
sperms are  found.  From  the  Cretaceous  upward 


150  Plant  Life  and  Evolution 

they  rapidly  increase  in  number  and  variety, 
and  many  existing  genera  can  be  plainly  recognized 
among  these  Cretaceous  fossils.  So  far  as  the  evi- 
dences of  geology  go,  the  two  great  divisions  of 
angiosperms — Monocotyledons  and  Dicotyledons — 
seem  to  be  of  about  equal  antiquity,  and  practically 
no  light  is  shed  on  the  relationships  of  these  two 
groups  to  each  other  from  a  study  of  their  fossil 
remains.  Nor  are  there  any  evidences  of  fossil 
remains  intermediate  between  angiosperms  and 
gymnosperms,  unless  some  of  the  cycad-like  forms 
of  the  Mesozoic  are  to  be  so  regarded,  and  this  at 
present  is  at  least  doubtful.  Equally  unsatisfactory 
are  the  attempts  to  derive  the  angiosperms  from 
any  of  the  existing  gymnosperm  types. 

The  Earlier  Fossil  Angiosperms. — The  sudden 
appearance  of  the  angiosperms  in  the  Sub-Cre- 
taceous formations,  and  the  close  resemblance  of 
these  earliest  fossils  to  living  forms,  makes  it  quite 
likely  that  the  earliest  angiosperms  have  left  no 
visible  traces  in  the  rocks.  This  may  be  due  either 
to  the  perishable  nature  of  these  primitive  forms, 
which  were,  perhaps,  delicate  herbaceous  plants,  like 
many  of  the  existing  monocotyledons,  or  such  low 
dicotyledons  as  the  buttercups  and  some  of  the  pep- 
pers; or  it  may  be  that  the  primitive  forms  lived 
in  relatively  dry  localities  where  the  conditions  for 
fossilization  were  not  favorable. 

The  Flower. — The  flower  of  the  angiosperms  is 
a  much  more  highly  developed  structure  than  that 


The  Angiosperms  151 

of  the  gymnosperms.    In  the  latter  the  resemblance 

of  the  sporophylls  to  the  spore-bearing  leaves  of 

A     „*••••••  «P 


FIG.  16 

A — Diagram  of  simple  pistil  of  an  Angiosperm,  composed 
of  a  single  carpel  or  sporophyll.  The  upper  part  forms  the 
"stigma,"  or  receptive  surface,  upon  which  the  pollen-spores, 
sp,  germinate.  The  pollen  tubes  traverse  the  elongated 
"  style,"  before  they  reach  the  ovule,  or  megasporangium, 
within  the  enlarged  cavity,  or  "  ovary,"  formed  by  the  base  of 
the  carpel.  Within  the  ovule  is  born  a  single  large  spore  (m), 
the  "  embryo-sac." 

B — Ripe  poilen-spore,  showing  large  sterile  cell,  and  anther- 
idial  cell,  an. 

C — The  megaspore,  or  embryo-sac,  shown  in  A,  m,  with  the 
enclosed  gametophyte  reduced  to  eight  cells.  At  the  upper 
end,  the  egg-apparatus  consisting  of  two  synergidse,  s,  and  the 
egg-cell,  o ;  at  the  base  three  "  antipodal "  cells,  a ;  in  the  mid- 
dle the  two  "  polar  "  nuclei,  p. 

the    pteridophytes    is    very   plain.     In   the    angio- 
sperms  the  sporophylls  are  very  much  more  altered. 


152  Plant  Life  and  Evolution 

The  microsporangia,  or  pollen-sacs,  are  borne  upon 
much  modified  sporophylls  called  stamens,  and  the 
leaves  known  as  carpels,  from  which  are  developed 
the  ovules,  or  megasporangia,  are  also  greatly  al- 
tered. The  carpels  form  a  closed  cavity,  the  ovary, 
within  which  the  ovules  are  placed  so  that  they  are 
effectively  protected  during  the  development  of  the 
seed.  The  flowers  also  possess  in  many  cases  showy 
leaves,  forming  the  floral  envelopes.  The  ovules 
are  not  strikingly  different  from  those  of  the  gym- 
nosperms,  but  there  is  usually  a  second  integu- 
ment. 

Origin  of  the  Angiosperms  Doubtful. — There  are 
undoubtedly  certain  points  in  common  between  the 
Gnetales  and  the  angiosperms,  this  being  especially 
the  case  in  the  genus  Gnetum;  but  as  we  have  al- 
ready seen,  the  Gnetales  are  very  doubtfully  related 
to  the  other  gymnosperms,  so  that  even  if  it 
should  be  shown  that  there  is  a  relation  between 
the  Gnetales  and  the  angiosperms,  this  would  not 
help  much  in  explaining  the  relation  of  the  latter 
to  lower  forms.  Some  of  the  monocotyledons,  es- 
pecially the  palms,  bear  a  superficial  resemblance 
to  the  cycads;  but  this  is  probably  only  a  paral- 
lelism, and  does  not  indicate  any  real  relation- 
ship. 

Gametophytes  of  the  Angiosperms The  pollen- 
spores  of  the  angiosperms  are  not  materially  dif- 
ferent in  structure  from  the  microspores  of  the 
heterosporous  pteridophytes  or  the  pollen-spores  of 


The  Angiosperms  153 

the  gymnosperms.  The  male  gametophyte  is  also 
very  similar  to  that  of  many  gymnosperms.  The 
very  slight  change  shown  in  the  evolution  of  the 
microspore  is  remarkable.  There  is  little  difference 
structurally  between  the  pollen-spore  in  the  highest 
seed-plant  and  the  spore  of  the  humblest  liver- 
wort. 

The  development  of  the  megaspore  shows  many 
analogies  with  that  of  the  gymnosperms,  and  the 
early  development  of  the  female  gametophyte  is 
very  similar.  The  early  stages  of  germination  in 
the  embryo-sac  are  quite  similar  to  those  in  the  pine 
or  Selaginella;  but  the  nuclear  divisions  are  much 
less  numerous,  so  that  in  the  normal  embryo-sac,  at 
the  time  it  is  fertilized,  there  are  only  eight  nuclei, 
representing  as  many  cells,  of  the  extremely  re- 
duced female  gametophyte;  or  in  some  cases  there 
may  be  only  seven  nuclei,  as  there  is  a  fusion  of  two 
of  the  original  nuclei  (Fig.  16,  C). 

There  has  been  much  discussion  as  to  the  homolo- 
gies  of  the  structures  of  the  embryo-sac  in  gymno- 
sperms and  angiosperms,  and  the  matter  is  still  in 
dispute.  The  most  marked  departures  from  the 
ordinary  angiospermous  type  are  found  in  two 
plants  which  are  in  some  respects  very  unlike.  In 
the  genus  Peperomia,  a  simple  dicotyledon  which 
in  some  ways  suggests  some  of  the  lower  mono- 
cotyledons, the  gametophyte  has  sixteen  instead  of 
eight  nuclei,  and  this  is  true  also  in  Gunnera;  and 
in  the  screw-pine  (Pandanus),  a  monocotyledon, 


154  Plant  Life  and  Evolution 

there  may  be  50  or  more  nuclei  at  the  time  of  fer- 
tilization. In  all  of  these  genera,  which  are  usually 
considered  to  be  primitive  types,  the  occurrence  of 
these  different  forms  of  embryo-sac  is  interesting, 
as  it  suggests,  at  least,  a  condition  intermediate 
between  the  embryo-sac  of  the  typical  angiosperms 
and  that  found  in  some  lower  type,  possibly  a  form 
like  Gnetum. 

It  must  be  borne  in  mind  that  this  microscopic 
body  within  the  embryo-sac,  composed  of  eight  cells, 
and  the  still  more  rudimentary  structure  developed 
from  the  germinating  pollen-spore,  are  all  that  re- 
main of  the  sexual  plants  or  gametophytes,  which 
in  the  lower  archegoniates  are  so  conspicuous.  In 
the  angiosperms  the  plant,  as  we  ordinarily  think 
of  it,  is  the  strictly  non-sexual  sporophyte.  The 
stamens  and  carpels  are  not  the  sexual  reproductive 
organs,  the  latter  being  really  represented  by  the 
rudimentary  archegonium  in  the  embryo-sac  and  the 
very  simple  antheridium  in  the  pollen-spore,  which 
correspond  to  the  sexual  organs  in  the  mosses  and 
ferns. 

Flowers  of  the  Lower  Angiosperms. — In  many 
of  what  are  usually  considered  the  most  primitive 
angiosperms,  e.g.,  willows,  poplars,  bur-reeds, 
screw-pines,  and  others,  the  flowers  are  diclinous, 
that  is,  stamens  and  carpels  are  in  different  flowers, 
and  often  upon  different  plants.  The  flower  may 
even  be  reduced  to  a  single  carpel  or  stamen,  and 
the  floral  envelopes,  which  are  so  striking  a  feature 


The  Angiosperms  155 

of  most  flowers,  may  be  quite  absent  or  represented 
only  by  inconspicuous  scales.  There  is  much  dif- 
ference of  opinion  as  to  the  nature  of  these  very 
simple  flowers.  Some  of  them  may  be  explained 
as  reductions  from  a  more  perfect  type;  but  such 
an  explanation  is  not  satisfactory  in  the  case  of 
other  forms,  and  most  botanists  believe  that  the 
simple  characters  of  many  of  these  "  apetalous  " 
flowers  are  really  primitive  (Figs.  17,  18). 

The  Typical  Angiospermous  Flower. — In  much 
the  greater  number  of  Angiosperms  the  flowers  are 
"  hermaphrodite,"  "  perfect,"  or  to  use  the  more  ac- 
curate term,  "  amphisporangiate,"  having  both 
stamens  and  carpels.  Moreover,  these  flowers  pos- 
sess usually  a  conspicuous  floral  envelope,  which 
may  consist  of  nearly  uniform  colored  leaves,  as  in 
the  lilies,  or  which  may  be  differentiated  into  a  green 
"  calyx  "  and  a  highly  colored  "  corolla."  There 
are  a  number  of  types,  both  among  monocotyledons 
and  dicotyledons,  in  which  the  flower  consists  of 
an  indefinite  number  of  quite  separate  parts,  the 
carpels  also  being  entirely  free  from  each  other. 
Such  flowers  are  known  as  "  apocarpous "  ones. 
The  magnolia,  buttercup,  and  arrow-head  are  exam- 
ples of  such  apocarpous  flowers  (Figs.  17,  B;  18, 
C).  These  flowers  show  a  repetition  of  parts,  espe- 
cially in  the  stamens  and  carpels,  while  in  the  more 
usual  floral  types  there  is  a  definite  number  of  parts 
with  a  tendency  towards  reduction  in  the  number 
of  stamens  and  carpels,  and  the  carpels  are  also 


156  Plant  Life  and  Evolution 

usually  united  into  a  compound  structure,  the  pistil 
(Fig.  i8,D). 

The  apocarpous  flowers  are  generally  admitted  to 
be  primitive  types,  but  whether  all  of  the  diclinous 
flowers  are  reduced  from  such  types,  having  both 
stamens  and  carpels,  is  another  question.  It  seems 
more  in  accordance  with  the  data  at  hand  to  assume 
that  in  the  primitive  angiospermous  stock  there  were 
developed  both  monosporangiate  and  amphisporan- 
giate  flowers.  From  these  two  types  of  flowers  in 
the  primitive  angiosperms,  the  similar  types  of  flow- 
ers, as  we  now  find  them  in  both  monocotyledons 
and  dicotyledons,  may  very  well  have  directly  arisen. 

Fertilization  in  Angiosperms. — The  peculiar  posi- 
tion of  the  ovules  of  the  angiosperms,  protected 
within  the  ovary,  involves  special  adaptations  for 
insuring  fertilization.  Instead  of  the  pollen  coming 
into  contact  directly  with  the  ovule,  as  it  does  in 
the  Gymnosperms,  it  falls  upon  the  variously  modi- 
fied upper  portion  of  the  pistil,  the  stigma 
(Fig.  1 6,  A).  This  is  specially  fitted  to  retain  the 
pollen  and  to  facilitate  its  germination,  but  the 
germination  of  the  pollen  itself  is  very  similar  to 
that  of  the  gymnosperms.  After  germination  the 
pollen-tube  must  traverse  the  whole  length  of  the 
pistil  before  it  finally  reaches  the  ovules.  The  inter- 
mediate portion  of  the  pistil,  or  "  style,"  is  usually 
cylindrical  in  form,  and  the  central  tissue  is  espe- 
cially modified  so  as  to  form  a  special  conducting 
tissue  which  nourishes  the  rapidly  growing  pollen- 


The  Angiosperms  157 

tube,  as  it  pushes  its  way  through  this  conducting 
tissue  until  it  finally  reaches  the  opening  of  the 
ovule,  which  it  enters  and  fertilizes  the  egg  in  much 
the  same  way  as  we  have  already  described  for  the 
gymnosperms.  The  highly  modified  structures  of 
the  carpels,  so  different  from  the  open  leaf-like  ones 
of  most  gymnosperms,  is  one  of  the  most  striking 
characters  of  the  angiospermous  flower. 

The  effect  of  fertilization  is  the  development  of 
the  embryo  from  the  egg  and  the  hardening  of 
the  outer  tissues  of  the  ovule,  which  now  be- 
comes a  seed,  resembling  very  closely  the  seed  of 
the  gymnosperms.  A  marked  difference  in  the 
seed  may  be  noted,  however.  The  "  endosperm," 
or  gametophyte  tissue  which  surrounds  the  em- 
bryo, and  which  in  the  gymnosperms  is  developed 
before  fertilization,  in  the  angiosperm  arises 
subsequent  to  fertilization,  and  is  the  result  of 
the  division  of  the  endosperm-nucleus,  which  is 
formed  by  the  fusion  of  the  two  polar  nuclei  (Fig. 
1 6,  C,  />).  The  latter  may  also  sometimes  fuse 
with  one  of  the  male  nuclei  from  the  pollen-tube. 
The  homologies  existing  between  the  endosperm  of 
the  gymnosperms  and  the  angiosperms  are  not, 
therefore,  entirely  clear.  It  has  even  been  claimed 
that  the  endosperm  of  the  angiosperms  is  rather 
in  the  nature  of  an  embryo  than  part  of  the  gameto- 
phyte, but  we  are  inclined  to  the  older  view  that  it 
really  belongs  to  the  gametophyte  as  it  does  in  the 
gymnosperms,  and  that  the  nuclear  fusion  which 


158  Plant  Life  and  Evolution 

precedes  the  formation  of  the  endosperm  is  not  to 
be  regarded  as  a  real  fertilization. 

The  Fruit. — The  effect  of  fertilization  extends 
beyond  the  ovule  itself,  and  the  carpels  within  which 
the  developing  seed  are  enclosed  are  also  stimulated 
into  growth  as  the  result  of  pollination,  and  at  ma- 
turity enclose  the  ripened  seed  within  a  "  fruit." 
It  is  probable  that  the  development  of  the  fruit, 
evidently  a  highly  important  means  for  protecting 
the  seed,  has  been  one  of  the  factors  in  the  success 
of  the  angiosperms  in  their  struggle  for  existence. 
The  variety  of  fruits  is  almost  as  great  as  that 
of  flowers,  and  it  is  quite  as  evident  in  many  cases 
that  the  modifications  of  the  fruit  are  associated 
with  animals,  which  play  an  important  part  in  the 
distribution  of  the  seeds  of  the  angiosperms.  Some 
of  the  modifications  of  fruits  which  are  pretty  cer- 
tainly concerned  with  their  distribution  through  ani- 
mal agencies,  are  the  development  of  edible  pulp, 
either  from  the  carpels  themselves  or  from  neigh- 
boring tissue,  and  the  development  of  grappling  or- 
gans, hooks,  or  spines,  or  occasionally  an  adhesive 
pulp,  as  in  the  fruit  of  the  mistletoe.  Many  fruits 
and  seeds  are  especially  adapted  for  distribution  by 
the  wind,  as  for  example  the  wind-borne  fruits  and 
seeds  of  the  willow,  catalpa,  dandelion,  and  ma- 
ple. There  is  no  doubt  that  these  many  successful 
methods  of  fruit  distribution  were  also  important 
factors  in  establishing  the  ascendency  of  the  angio- 
sperms. 


The  Angiosperms  159 

MONOCOTYLEDONS  AND  DICOTYLEDONS 

A  survey  of  the  multitude  of  angiosperms  which 
everywhere  abound  shows  two  pretty  well-marked 
series  of  forms.  One  series  may  be  represented  by 
a  tulip  or  daffodil,  herbaceous  plants  having  smooth, 
elongated  leaves  with  inconspicuous,  unbranched, 
parallel  veins,  and  flowers  with  the  parts  in  sets  of 
three,  the  two  outer  sets  of  leaves  being  alike.  If 
the  seedling  plant  is  examined,  it  will  be  found  to 
possess  a  single  seed-leaf  or  "  cotyledon  " ;  hence 
these  plants  are  called  "  Monocotyledons." 

Mustard  may  be  taken  as  a  type  of  the  second 
series.  The  seedling  shows  two  opposite  cotyledons ; 
hence  the  name  "  Dicotyledon  "  applied  to  the  series. 
The  young  plant  increases  rapidly  in  size,  and  an 
examination  of  the  stem  structure  shows  that  the 
woody  bundles  of  the  stem  continue  to  increase  in 
size  by  the  addition  of  new  tissue  derived  from  a 
layer  of  permanently  growing  tissue,  or  cambium, 
which  results  in  a  secondary  thickening  of  the 
stem.  This  in  the  woody-stemmed  shrubs  and  trees 
exhibits  annual  growth  rings  like  those  in  the  trunk 
of  a  pine  or  other  coniferous  tree.  No  monocoty- 
ledons show  this  type  of  secondary  growth  in  thick- 
ness. 

The  leaves  in  the  mustard  are  more  or  less  deeply 
divided  or  cut,  and  their  veins  are  much  branched 
and  more  or  less  united  into  an  elaborate  network. 
The  flowers  have  the  parts  in  fours,  although  the 


160  Plant  Life  and  Evolution 

outer  set  of  stamens  and  the  carpels  are  reduced  to 
two.  The  outer  floral  leaves  are  differentiated  into 
a  calyx  or  outer  envelope,  and  a  corolla,  the  inner 
highly  colored  set  of  floral  leaves. 

When  we  try  to  determine  the  relative  rank  of 
these  two  series,  we  are  met  by  much  the  same 
difficulties  that  we  encounter  in  attempting  to  trace 
the  connection  of  the  angiosperms  as  a  whole  with 
the  gymnosperms  or  pteridophytes.  So  far  as  the 
geological  record  goes,  the  two  types  seem  to  have 
developed  almost  simultaneously.  It  was  formerly 
supposed  that  certain  Paleozoic  and  early  Mesozoic 
impressions  of  leaves  belonged  to  monocotyledons, 
and  that  the  greater  antiquity  of  the  latter  was  es- 
tablished ;  but  much  doubt  has  been  thrown  upon  the 
nature  of  these  fossils,  and  they  are  now  generally 
considered  to  be  remains  either  of  Cordaitales  or 
cycads.  As  the  great  majority  of  living  monocoty- 
ledons are  herbaceous  plants,  often  of  very  delicate 
texture,  it  is  by  no  means  unlikely  that  they  may 
have  existed  earlier  than  is  indicated  by  the  fossil 
record,  and  the  same  may  be  said  of  some  of  the 
lower  types  of  dicotyledons,  although  in  the  latter 
group,  many  of  what  are  regarded  as  primitive 
types  are  trees,  like  the  willow,  poplar,  and  plane, 
and  these  are  among  the  earliest  fossil  angiosperms 
that  we  know. 

Fossil  Angiosperms. — There  seems  to  be  no  rea- 
sonable doubt  (see  Zeiller :  "  Elements  de  Paleobo- 
tanique,"  1900)  that  remains  of  both  monocoty- 


The  Angiosperms  161 

ledons  and  dicotyledons  occur  in  the  Infra- 
Cretaceous  rocks ;  but  the  evidence  of  the  occurrence 
of  either  group  in  earlier  formations  is  extremely 
doubtful.  If  we  try  to  determine  the  question  from 
a  study  of  the  living  forms,  the  matter  is  equally 
difficult.  Among  both  monocotyledons  and  dicoty- 
ledons, flowers  of  equal  simplicity  are  met  with. 
Such  monocotyledons  as  the  arums,  pond-weeds, 
or  screw-pines  may  be  compared  to  such  apetalous 
dicotyledons  as  the  peppers  or  willows;  while  the 
primitive  type  of  amphisporangiate  flowers  with  its 
indefinitely  multiplied  free  parts,  like  the  magnolia 
and  buttercup,  occurs  also  in  the  water-plantain 
and  some  other  low  monocotyledons. 

There  is  no  doubt  that  as  a  whole  the  monocoty- 
ledons are  simpler  than  the  dicotyledons,  but  the 
question  then  arises,  whether  this  greater  sim- 
plicity is  not  in  many  cases  secondary,  the 
result  of  reduction.  The  same  difficulty  is  again 
met  with  in  comparing  the  embryo,  whose  char- 
acter gives  name  to  the  two  series.  While  most 
dicotyledons,  as  their  name  implies,  have  embryos 
with  two  cotyledons,  there  are  some  in  which 
but  one  is  present,  and  the  question  whether 
the  monocotyledonous  or  dicotyledonous  condition 
is  more  ancient  still  remains  open.  It  is  quite  con- 
ceivable that  the  two  divisions,  as  usually  recognized, 
do  not  necessarily  represent  two  closed  develop- 
mental series,  and  whether  we  consider  monocoty- 
ledon or  dicotyledon  as  the  more  primitive  type, 


162  Plant  Life  and  Evolution 

it  is  not  impossible  that  from  this  primitive  group 
more  than  one  line  leading  to  the  other  may  have 
arisen. 

Recent  Theories  as  to  Origin  of  Angiosperms 

Several  recent  writers  have  argued  for  the  deriva- 
tion of  all  the  angiosperms  from  types  allied  to 
some  of  the  Mesozoic  Cycadales,  in  which  the  sporo- 
phylls  are  arranged  very  much  as  they  are  in  the 
living  magnolia.  It  is  also  argued  that  the  gym- 
nosperms  and  pteridophytes  show,  for  the  most 
part,  structures  approximating  the  dicotyledonous 
type,  rather  than  that  of  the  monocotyledons,  and, 
moreover,  that  the  embryo  in  the  gymnosperms  is 
usually  dicotyledonous.  From  this  hypothetical 
dicotyledonous  ancestral  form,  with  presumably 
woody  stem  and  amphisporangiate  flowers,  types 
with  the  simpler  monosporangiate  flowers,  and 
herbaceous  habit,  would  be  derived  by  reduction. 

There  may,  however,  be  said,  on  the  other  side, 
that  most  of  the  gymnosperms  and  pteridosperms 
are  monosporangiate,  and  primitively  so.  More- 
over, the  embryo  of  the  ferns,  and  the  same  is  true 
of  the  peculiar  aquatic  heterosporous  pteridophyte 
Isoetes,  which  in  many  ways  is  strikingly  similar  in 
habit  to  the  monocotyledons,  is  monocotyledonous ; 
so  it  will  be  seen  that  something  may  be  said  for  the 
assumption  that  the  primitive  angiosperms  were 
monocotyledons  and  monosporangiate.  It  seems  to 
the  writer  that  the  evidence  available  indicates  that 
the  two  types  of  flower — the  monosporangiate  type, 


The  Angiosperms  163 

such  as  occurs  in  the  screw-pines  and  willows,  and 
the  amphisporangiate  type  of  the  magnolia  and 
water-plantain,  may  be  equally  primitive,  and  not 
derived  one  from  the  other. 

Whether  these  two  types  first  arose  in  mono- 
cotyledons or  dicotyledons  must  remain  for  the 
present  an  open  question.  It  is  quite  likely  that 
the  primitive  angiosperms  were  not  clearly  divided 
into  monocotyledons  and  dicotyledons  as  they  are 
now  understood.  From  this  primitive  stock  pre- 
sumably more  than  two  series  arose,  and  these  may 
have  started  with  either  type  of  flower.  Thus  at 
present  some  of  the  so-called  Ranales,  i.e.,  butter- 
cups, water-lilies,  etc.,  are  dicotyledonous  forms 
which  are  almost  certainly  directly  related  to  such 
monocotyledonous  types  as  the  water-plantain  and 
pond-weeds;  while  we  believe  a  similar  relationship 
exists  between  the  monocotyledonous  arums  and  the 
dicotyledonous  peppers.  Thus,  while  it  is  reason- 
ably certain  that  all  of  the  angiosperms  belong  to 
a  common  stock,  it  by  no  means  follows  that  there 
may  not  have  been  developed  from  this  more  than 
one  line  both  of  monocotyledons  and  dicotyledons. 

MONOCOTYLEDONS 

Whether  or  not  the  monocotyledons  are  more 
primitive  than  the  dicotyledons,  there  is  no  question 
that  as  a  class  they  are  decidedly  simpler,  both  in 
their  external  form  and  in  their  tissues.  As  their 


164  Plant  Life  and  Evolution 

name  indicates,  the  embryo  is  provided  with  a  single 
primary  leaf,  or  cotyledon,  and  is  thus  easily  dis- 
tinguished from  that  of  most  dicotyledons,  some 
species  of  which,  however,  have  monocotyledonous 
embryos. 

As  a  rule,  the  monocotyledons  are  herbaceous 
plants,  very  often  having  their  leafy  shoots  arising 
from  special  permanent  underground  structures, 
root-stocks,  bulbs,  tubers,  etc.,  and  this  "  geoph- 
ilous,"  or  underground,  habit  of  the  stem  has  been 
assumed  to  be  an  adaptation  which  accounts  to  some 
extent  for  the  reduced  character  of  many  mono- 
cotyledons, but  this  is  an  hypothesis  which  requires 
further  demonstration  before  it  can  be  accepted 
without  question.  The  leaves,  as  a  rule,  are  sim- 
ple, smooth-margined,  with  parallel  venation,  but 
many  arums  and  some  lilies  have  net-veined 
leaves,  like  the  dicotyledons.  Very  few  of  the 
monocotyledons  are  trees,  the  most  marked  excep- 
tions being  the  palms  and  some  screw-pines,  al- 
though even  among  the  lilies  there  are  certain 
genera,  like  Yucca  and  Dracaena,  which  may  attain 
the  dimensions  of  small  trees.  No  monocotyledon, 
however,  shows  the  type  of  secondary  thickening 
of  the  trunk,  which  is  so  common  in  dicotyledons, 
and  in  the  few  cases  where  there  is  a  secondary 
thickening  of  stem  it  is  of  quite  a  different  char- 
acter. 

The  absence  of  cambium  from  the  woody  bun- 
dles of  the  stem  is  a  constant  feature  in  mono- 


The  Angiosperms  165 

cotyledons,  but  it  has  been  claimed  that  traces  of 
such  thickening  tissues  have  been  found  in  the  seed- 
lings of  many  monocotyledons,  indicating  that  its 
absence  in  the  woody  bundles  of  the  mature  plant 
is  a  secondary  condition.  While  the  monocoty- 
ledons are  as  a  rule  less  conspicuous  than  the  di- 
cotyledons in  the  flora  of  most  regions,  there  are 
certain  types  which  are  gregarious  and  predominate 
over  large  areas.  This  is  conspicuously  true  of  the 
grasses  in  open  prairie  country ;  and  in  swampy 
districts,  rushes,  sedges,  etc.,  may  be  found  to  be 
the  principal  constituents  of  the  vegetation,  and 
under  these  conditions  are  evidently  quite  able  to 
hold  their  own  in  competition  with  the  usually  more 
aggressive  dicotyledons. 

Monocotyledonous  Flowers. — The  flowers  of  the 
Monocotyledons,  as  we  have  already  seen,  belong 
to  the  two  principal  types,  apetalous  and  petaloid- 
eous.  Of  the  former,  some  of  the  types  cannot 
readily  be  explained  as  reductions  from  petaloideous 
flowers,  and  they  are  in  all  probability  really  primi- 
tive types.  Such,  for  example,  are  the  flowers  of  the 
cattail  rushes,  the  bur-reeds,  and  the  screw- 
pines.  The  two  last,  in  addition  to  the  simple  type 
of  flowers,  show  marked  indications  of  the  more 
primitive  condition  of  the  gametophyte,  which  is 
much  better  developed  than  in  most  angiosperms. 
The  arums  and  palms  are  also  probably  old  types, 
not  reduced  from  petaloideous  forms,  but  as  in  these 
two  families  both  hermaphrodite  and  diclinous  flow- 


:i66  Plant  Life  and  Evolution 

ers  occur,  it  is  a  question  which  of  these  two  types 
is  the  older. 

The  petaloideous  monocotyledons  constitute  the 
second  category,  in  most  of  which  the  floral  envel- 
opes are  conspicuous,  and  these  include  some  of  the 
most  beautiful  and  highly  specialized  of  all  flowers. 
The  simplest  types  are  seen  in  the  lilies,  e.g.,  tu- 
lip, hyacinth,  trillium,  etc.  They  have  the  char- 
acteristic three-fold  arrangement  of  the  floral  or- 
gans with  all  the  parts  separate  except  the  three 
united  carpels;  but  the  leaves  of  the  floral  envelope 
may  be  more  or  less  united,  as  in  the  hyacinth  or 
lily-of-the-valley.  This  appears  to  be  the  central 
type  from  which  have  radiated  several  lines  of  de- 
velopment, resulting  in  flowers  differing  a  good  deal 
from  the  primitive  lily  type. 

The  deviations  from  the  primitive  type  are  two- 
fold. First  there  is  a  reduction  in  the  number  of 
stamens,  and  second,  a  more  or  less  complete  fusion 
of  certain  parts  of  the  flower.  The  simplest  case  is 
seen  in  the  Amaryllis  family,  of  which  the  narcissus 
and  snowdrop  are  common  examples.  In  these  flow- 
ers the  base  of  the  floral  envelope,  and  probably  to 
some  extent  the  floral  axis,  are  fused  with  the  lower 
parts  of  the  carpels,  so  that  there  is  formed  an  "  in- 
ferior "  ovary.  Otherwise,  the  flowers  are  like  those 
of  the  lily.  In  all  of  the  higher  types  of  monocoty- 
ledonous  flowers  there  is  an  inferior  ovary.  In  the 
Iris  family  there  is  a  reduction  of  stamens  to  three, 
and  a  strong  tendency  to  replace  the  radial  symmetry 


The  Angiosperms  167 

of  the  flower  by  a  bilateral  symmetry,  as  in  Gladi- 
olus (Fig.  17,  C).  In  the  Canna  only  one  stamen 
is  perfect,  and  the  others -are  modified  into  petal- 


FIG.  17 

Types  of  Monocotyledonous  Flowers. 
A — Pistillate  flower  of  Bur  reed  (Sparganium). 
B — Staminate  flower  of  Arrow-head  (Sagittaria). 
C — Flower  of  Gladiolus.     The  flower  is  "  Zygomorphic,"  the 
stamens  reduced  to  three,  and  there  is  an  "  interior  "  ovary,  o. 

like  organs,  while  the  flower  is  strongly  zygomor- 
phic,  i.e.,  is  bilaterally  symmetrical. 

It  is  among  the  orchids,  however,  that  we 
find  the  most  highly  specialized  type  of  flowers 
among  the  monocotyledons.  This  great  family, 
the  largest  one  among  the  monocotyledons,  has 
the  stamens  generally  reduced  to  a  single  one, 


1 68  Plant  Life  and  Evolution 

which  is  more  or  less  completely  fused  with  the 
upper  part  of  the  carpel,  forming  a  peculiar  struc- 
ture known  as  the  column,  which  distinguishes  the 
flowers  of  the  orchids  (Fig.  22,  B).  The  flowers 
are  strongly  zygomorphic,  and  like  most  highly  spe- 
cialized flowers  they  are,  as  a  rule,  quite  depend- 
ent upon  insect  aid  for  pollination.  The  orchids, 
which  constitute  the  second  largest  family  of  flow- 
ering plants,  show  the  highest  degree  of  specializa- 
tion found  among  the  monocotyledons,  and  also 
exhibit  remarkable  plasticity,  as  they  grow  under  a 
great  variety  of  conditions.  There  are  many  epi- 
phytic orchids,  or  "  air  plants,"  and  a  good  many 
which  have  lost  their  chlorophyll  and  become  sapro- 
phytic,  living  upon  the  decayed  organic  matter  in 
leaf  mold.  The  orchids,  however,  do  not  as  a  rule 
seem  to  be  capable  of  adapting  themselves  readily  to 
new  conditions,  and  never  seem  able  to  compete  suc- 
cessfully with  the  more  aggressive  plant  types  when 
they  are  removed  from  their  usual  environment. 
They  give  the  impression  of  a  group  of  plants  in 
the  process  of  evolution,  making  experiments  in 
various  directions,  and  developing  a  great  variety 
of  types,  most  of  which,  however,  are  more  or  less 
pronounced  failures. 

DICOTYLEDONS 

Dicotyledons   the   Highest   of  All   Plants. — The 

angiosperms,  in  the  dicotyledons,  reach  their  most 


The  Angiosperms  169 

perfect  expression,  and  these  may  be  considered  to 
represent  the  highest  achievement  of  the  plant  type. 
Both  in  their  superior  numbers  and  adaptability 
they  show  themselves  to  be,  as  a  class,  better  fitted 
to  existing  conditions  than  do  any  other  class  of 
plants,  and  probably  among  them  are  the  most  recent 
plant  types  that  have  been  evolved.  They  include 
plants  adapted  to  practically  all  conditions  of 
life,  and  in  one  respect  only  are  they  surpassed  by 
the  monocotyledons,  namely  in  their  adaptation  to 
a  strictly  aquatic  condition.  The  number  of  truly 
aquatic  types  is  relatively  small,  although  some  of 
them,  like  the  bladder-weeds,  are  especially  adapted 
to  aquatic  life,  and  one  peculiar  group,  the  Podo- 
stemonaceae,  have  been  so  modified  as  to  look  more 
like  algae  than  flowering  plants.  No  forms  are 
known  which  live  in  sea  water,  the  nearest  approach 
to  this  being  certain  salt-marsh  plants,  and  the  man- 
groves. 

They  may  be  considered,  as  a  class,  to  be 
more  decidedly  terrestrial  in  habit  than  are  the 
monocotyledons.  From  tiny  herbs,  living  but  a  few 
weeks,  to  giant  trees,  living  as  many  centuries,  di- 
cotyledons are  everywhere  encountered,  and  usually 
in  greater  numbers,  both  of  individuals  and  species, 
than  are  the  monocotyledons.  Except  for  certain 
coniferous  trees,  the  dicotyledons  make  up  the  for- 
ests of  temperate  climates ;  and  with  the  exception 
of  the  palms,  they  may  be  said  also  to  constitute  the 
great  bulk  of  the  tropical  forests.  The  tendency 


170  Plant  Life  and  Evolution 

to  develop  permanent  woody  stems  is  a  marked  char- 
acter of  the  class,  and  this  is  due  to  the  development 
of  cambium,  or  permanent  growing  tissue,  in  the 
woody  bundles  of  the  stem,  very  much  as  in  the 
conifers,  and  in  the  stems  of  some  ancient  pterido- 
phytes.  This  gives  them  the  advantage  of  a  per- 
manent growth  of  the  stems,  which  do  not  have  to 
be  formed  anew  each  year,  as  is  so  commonly  the 
case  with  monocotyledons. 

With  the  greater  diversity  of  the  stem  structure 
goes  an  equally  diversified  leaf  type.  The  leaves 
may  be  either  simple  or  divided,  and  in  many  cases 
elaborately  dissected  in  a  variety  of  ways.  The 
great  adaptability  of  the  dicotyledonous  type  is  well 
shown  by  the  history  of  various  weeds.  While  some 
of  the  weeds  are  monocotyledons,  the  grasses  rank- 
ing first  in  this  category,  many  more  weeds  are 
dicotyledons,  which  often  have  been  brought  from 
remote  lands  but  have  very  quickly  made  them- 
selves at  home,  not  infrequently  driving  out  the 
native  flora.  Climbing  plants,  carnivorous  plants, 
epiphytes,  parasites,  and  aquatics  are  some  of  the 
types  that  have  been  evolved  among  the  dicoty- 
ledons. 

Flowers  of  Dicotyledons. — The  flowers  of  the 
dicotyledons  (Fig.  18)  are  far  too  various  in  their 
structure  to  permit  of  anything  more  than  the  brief- 
est sketch  of  their  more  salient  features.  Like  the 
monocotyledons  there  are  two  main  categories,  apet- 
alous  and  petaloideous  forms.  A  number  of  very 


The  Angiosperms 


171 


characteristic  orders  of  no  very  close  'affinity,  how- 
ever, are  often  grouped  under  the  head  of  Apetalae. 
Some  of  these,  both  upon  structural  grounds  and 


FIG.  18 


Types  of  Dicotyledonous  Flowers. 
A — Pistillate  flowers  of  Walnut. 
B — Staminate  flower  of  Poplar. 
C — Section  of  apocarpous  flower  of  Strawberry. 
D— Choripetalous  flower  of  Spring-beauty  (Claytonia). 
E — Sympetalous,    zygomorphic   flower   of    Cardinal    flower 
(Lobelia). 

from  geological  evidence,  are  evidently  ancient 
types,  and  there  is  no  good  reason  to  consider  them 
as  reduced  from  petaloideous  forms.  Such,  for  ex- 
ample, are  the  willows,  poplars,  oaks,  and  wal- 
nuts. These  all  have  diclinous  flowers,  and  it  is 
difficult  to  say  from  what  amphisporangiate  forms 
they  could  possibly  have  been  derived.  The  flower 
consists  either  of  stamens  or  carpels,  and  the  floral 


172  Plant  Life  and  Evolution 

envelope,  if  present  at  all,  is  of  the  simplest  char- 
acter (Fig.  1 8,  A,  B). 

The  petaloideous  forms  show  great  variety,  and 
it  is  not  easy  to  reduce  them  all  to  one  series. 
The  type  of  the  flower  found  in  the  buttercup  or 
magnolia  is  regarded  as  primitive,  but  it  is  difficult 
to  connect  these  forms  with  such  apetalous  flowers 
as  those  of  the  poplar  or  oak,  for  example.  There 
are  other  simple  petaloideous  types,  such,  for  exam- 
ple, as  the  pinks,  portulacas,  etc.,  which  are  quite 
different  from  such  apocarpous  types  as  the  butter- 
cup or  water-lily.  These  lower  petaloideous  types 
have  been  called  Choripetalse,  and  have  the  petals 
entirely  separate.  The  number  of  carpels  and 
stamens  may  be  quite  indefinite  or  they  may  be  of 
definite  number. 

The  specialization  of  the  flower  in  the  dicotyle- 
dons has  proceeded  very  much  in  the  same  way 
as  among  the  monocotyledons,  and  affords  an- 
other example  of  parallel  development.  There  is 
first  an  indefinite  number  of  entirely  separate  parts, 
then  a  reduction  in  the  number  of  stamens  and 
carpels,  and  a  tendency  toward  a  coherence  of  cer- 
tain parts,  resulting  in  a  more  or  less  tubular  flower, 
with  inferior  ovary  and  a  reduced  number  of  sta- 
mens. Where  the  petals  are  grown  together  the 
flower  is  said  to  be  "  sympetalous,"  and  the  Sym- 
petalae,  which  include  all  such  flowers,  are  consid- 
ered by  botanists  to  be  the  most  highly  specialized 
of  the  dicotyledons  (Fig.  18,  E;  Fig.  19,  F). 


The  Angiosperms  173 

Among  the  Choripetalse  there  are  much  greater 
differences  of  structure  than  among  the  Sym- 
petalae,  which  are  reducible  to  a  comparatively 
small  number  of  types,  although  in  point  of  num- 
bers they  probably  surpass  the  Choripetalse.  It 
is  among  these  highly  specialized  Sympetalae  that 
we  meet  with  the  most  successful  types,  these  being 
the  dominant  dicotyledons,  especially  in  the  tropics. 

The  largest  family  of  angiosperms  and  the  one 
which,  on  the  whole,  seems  to  have  succeeded  best 
in  the  struggle  for  life,  is  the  Composite.  The  uni- 
versal distribution  of  Compositae  and  the  aggressive 
character  of  many  of  them  are  sufficient  proofs  of 
the  efficiency  of  this  type.  This  superiority  seems 
to  be  more  or  less  due  to  their  extraordinarily  per- 
fect devices  for  the  transportation  of  their  seeds. 
The  numerous  wind-borne  seeds  of  the  dandelion 
and  thistles,  the  tenacious  burs  of  the  bur-marigold 
and  burdock,  together  with  the  robust  constitution 
of  the  plants  themselves,  have  given  these  weeds  an 
enormous  advantage  in  the  struggle  for  existence; 
and  we  see  them  scattered  over  vast  tracts  of  coun- 
try, taking  possession  of  the  vacant  ground  almost  to 
the  exclusion  of  the  plants  originally  inhabiting 
them. 

Evolution  of  the  Flower  in  Monocotyledons  and 
Dicotyledons  Much  Alike. — The  evolution  of  the 
flower  has  followed  very  much  the  same  course  in 
monocotyledons  and  dicotyledons,  and  illustrates 
once  more  the  remarkable  similarity  that  may  result 


174 


Plant  Life  and  Evolution 


from  response  to  similar  conditions  in  independent 
developmental  lines  (Fig.  19).  This  would  seem  to 
be  an  excellent  illustration  of  determinate  variation 


FIG.  19 

Parallelism  in  the  evolution  of  the  flower  in  Monocotyledons 
(upper  row)  and  Dicotyledons  (lower  row). 

A — Apetalous  flower  of  a  Sedge  (Carex). 

B — Apetalous  flower  of  a  Mulberry  (Morus). 

C — Apocarpous  flower  of  Water-plantain   (Alisma). 

D — Apocarpous  flower  of  Rue- Anemone  (Thalictrum). 

E — Zygomorphic  sympetalous  flower  of  an  Orchid 
(Arethusa). 

F — Zygomorphic  sympetalous  flower  of  Dead-nettle 
(Lamium). 

(Fig.  C,  after  Britton  &  Brown.) 

in  several  lines  starting  from  a  common  stock  and 
resulting  in  very  similar  structures  at  the  end  of 
these  diverging  lines  of  development.  In  both  of  the 
great  divisions  of  the  angiosperms,  the  lower  types 


The  Angiosperms  175 

of  flowers,  whether  monosporangiate  or  amphi- 
sporangiate,  have  all  the  parts  entirely  separate  and 
often  indeterminate  in  number.  These  simple  types 
are  often  inconspicuous  and  are  probably  all  capa- 
ble of  self-pollination,  supposing  that  they  are 
amphisporangiate.  Where  the  flowers  are  diclinous, 
and  often  when  they  are  amphisporangiate,  as  in 
most  grasses,  the  distribution  of  the  pollen  is  almost 
always  effected  by  the  wind.  With  the  increase  in 
specialization,  the  flower  first  assumes  a  definite 
number  of  parts,  and  there  is  a  tendency  towards 
cohesion  of  its  parts,  which  is  usually  seen  first  in 
carpels.  Such  floral  types  as  the  lilies  and  mus- 
tards are  examples  of  this.  In  the  typical  mono- 
cotyledonous  flowers  the  carpels  are  equal  in  num- 
ber to  the  other  cycles  of  floral  leaves,  but  in  most 
of  the  dicotyledons  the  number  of  carpels  is  re- 
duced. There  are,  however,  numerous  types  among 
the  simpler  dicotyledons  where  the  carpels  are  also 
equal  in  number  to  the  other  parts  of  the  flower, 
and  these  are  said  to  be  "  isocarpous."  The  gera- 
nium and  flax  may  be  cited  as  examples  of  such 
isocarpous  flowers. 

The  cohesion  of  the  floral  parts  increases  with 
the  specialization  of  the  flower,  and  this  may  even 
extend  to  the  stamens  as  well  as  the  corolla.  Thus 
in  some  of  the  Pea-family  the  stamens  are  grown  to- 
gether into  a  tube,  and  the  same  is  seen  in  the  mal- 
lows. The  carpels  in  the  greater  number  of  angio- 
sperms  are  grown  more  or  less  together  and  form 


176  Plant  Life  and  Evolution 

a  "  syncarpous,"  or  compound  pistil.  The  lower 
part  of  the  pistil,  or  ovary,  usually  clearly  shows  the 
number  of  carpels  of  which  it  is  composed  either  by 
being  divided  into  chambers  or  having  a  correspond- 
ing number  of  placentae,  from  which  the  ovules 
grow.  The  upper  part  of  the  floral  axis  is  frequently 
extended  into  a  tube  at  the  bottom  of  which  the 
ovary  lies,  and  in  very  many  flowers  both  Chori- 
petalse  and  Sympetalse,  the  lower  portion  of  the  tube 
may  be  completely  fused  with  the  ovary,  and  the 
latter  is  then  said  to  be  inferior,  the  other  parts  of 
the  flower  being  "  epigynous."  Such  epigynous 
flowers  are  considered  to  be  more  specialized  than 
those  in  which  the  ovary  is  free.  The  iris,  nar- 
cissus, canna,  and  the  orchids  are  typical  examples 
of  epigynous  monocotyledons,  while  the  fuchsia 
and  the  Compositae  are  illustrations  of  epigyny  in 
the  dicotyledons. 

Sympetaly,  or  the  union  of  the  petals,  is  a 
common  phenomenon  in  both  the  monocotyledons 
and  dicotyledons.  It  reaches  its  extreme  in  such 
flowers  as  the  morning-glories  or  the  fox- 
glove, where  the  limits  of  fhe  individual  segments 
or  petals  are  almost  obliterated.  Finally  in  the 
orchids  there  is  an  almost  complete  coalescence  of 
the  carpels  and  stamens,  the  latter  being  reduced 
in  most  cases  to  a  single  one. 

The  Most  Specialized  Flowers  are  Zygomorphic. 
— In  the  more  primitive  flower  the  parts  are  usually 
arranged  radially,  but  in  many  flowers  the  sym- 


The  Angiosperms  177 

metry  is  bilateral,  the  flower  often  being  two-lipped, 
as  in  the  sage  and  other  "labiate"  flowers  (Fig. 
19,  E,  F).  This  "  zygomorphy  "  is  frequently  asso- 
ciated with  a  reduction  in  the  number  of  stamens, 
as  in  the  fox-glove  or  snap-dragon,  where  instead 
of  the  five  stamens  which  would  correspond  to  the 
number  of  petals,  there  are  but  four.  Sometimes  a 
rudiment  of  the  fifth  stamen  is  visible,  as  in  the 
Pentstemon.  Zygomorphy  does  not  necessarily  in- 
volve a  reduction  of  the  stamens,  the  sweet-pea,  for 
instance,  having  ten  stamens,  or  twice  the  number  of 
the  petals. 

Perfume  and  Color  as  Lures  for  Insects. — The 
inconspicuous  flowers  of  the  Apetalse  are  usually 
destitute  of  perfume,  which  is  so  marked  in  so  many 
flowers.  There  seems  no  good  reason  to  doubt  that 
the  presence  of  strong  odors,  agreeable  or  other- 
wise, is  associated  with  the  visits  of  insects  which 
are  attracted  to  the  flower  both  for  the  sake  of  the 
pollen,  which  was  probably  their  first  object  in  visit- 
ing the  flowers,  and  for  the  sake  of  the  nectar  which 
is  secreted  by  many  of  them.  The  gay  colors  of 
the  petals  and  sometimes  of  the  other  parts  of  the 
flowers,  or  inflorescence,  e.g.,  the  stamens  in  Euca- 
lyptus, the  sepals  in  the  Clematis,  or  the  accessory 
bracts  in  the  arum,  or  dogwood,  are  usually  also  re- 
garded as  a  means  of  attracting  insects  or  birds. 

There  has  been  lately  a  tendency  to  minimize  the 
importance  of  insects  as  the  agents  of  cross-pollina- 
tion, and  the  significance  of  the  coloring  of  the  flow- 


178  Plant  Life  and  Evolution 

ers  in  attracting  these,  and  it  is  quite  possible  that 
the  assumption  of  the  keen  power  of  discrimination 
of  different  colors  and  markings  which  has  been 
attributed  to  insects  has  been  exaggerated;  but  the 
evidence  is  overwhelming  that  there  is  a  direct  con- 
nection between  the  development  of  showy  flowers, 
and  cross-pollination  through  insect  agency. 

Cross-pollination. — While  many  showy  flowers, 
when  insect  visits  are  prevented,  pollinate  them- 
selves, there  are  very  many  in  which  cross-pollina- 
tion is  absolutely  indispensable  owing  to  mechanical 
contrivances  by  which  self-pollination  is  rendered 
impossible.  Some  of  these  will  be  discussed  more 
at  length  in  a  future  chapter.  Where  specializa- 
tion reaches  its  extreme,  pollination  may  depend 
upon  a  single  species  of  insect,  as  for  instance,  in 
certain  orchids  and  species  of  Yucca. 

Insects  as  Agents  in  Pollination. — One  group  of 
animals  has  played  a  very  important  role  in  the  evo- 
lution of  the  flower  of  the  angiosperms.  These 
are  the  insects,  the  largest  group  of  animals,  bearing 
somewhat  the  same  relation,  in  point  of  numbers, 
to  the  animal  kingdom  that  the  angiosperms  do  to 
plants.  Vast  numbers  of  insects  are  dependent  upon 
plants  for  their  existence,  and  many  of  the  peculiar 
modifications  of  their  structures  are  unquestionably 
correlated  directly  with  the  structures  of  angio- 
spermous  plants;  and  the  modifications  of  the  flow- 
ering plants  and  insects  have  presumably  gone  on 
side  by  side,  each  affecting  the  other.  Thus  the  pe- 


The  Angiosperms  179 

culiar  mouth-parts  of  the  flower-haunting  insects, 
like  the  bees  and  butterflies,  unquestionably  owe  their 
existence  to  the  peculiar  structures  of  the  flowers 
they  visit,  and  the  flowers  become  adapted  to  the 
associated  structures  of  the  insects.  The  enor- 
mously long  proboscides  of  the  big  hawk-moths  are 
only  to  be  explained  as  organs  especially  fitted  for 
probing  the  deep  nectaries  of  certain  flowers,  and 
the  pollen-receptacles  of  the  bee  must  have  been  de- 
veloped in  connection  with  the  habit  of  collecting 
pollen  for  food.  It  has  often  been  claimed  that  the 
peculiar  formation  of  many  flowers  is  the  direct 
reaction  to  stimuli,  due  to  the  irritation  of  special 
parts  of  the  flower  during  the  visits  of  insects.  It 
may  be  said,  however,  that  this  view  is  not  generally 
accepted. 

Birds  as  Agents  in  Pollination. — Many  flowers 
are  adapted  to  fertilization  by  birds,  which  have  be- 
come modified  accordingly.  The  great  American 
family  of  humming-birds,  and  the  honey-suckers  of 
the  Old  World,  are  the  best-known  types.  These 
two  groups  of  birds,  although  not  at  all  related, 
show  curiously  similar  characters  in  size,  color,  and 
form,  and  the  flowers  they  frequent,  both  in  shape 
and  color,  show  a  corresponding  similarity.  Bright 
red  seems  to  be  the  favorite  color  of  these  much  fre- 
quented flowers,  and  in  America  many  vivid  red 
flowers,  like  the  trumpet-creeper,  the  scarlet  balm, 
scarlet  sage,  trumpet  honeysuckle,  and  many 
others  may  be  mentioned  as  special  favorites  of 


180  Plant  Life  and  Evolution 

humming-birds;  while  in  South  Africa,  the  scarlet 
Aloes  and  coral  trees  (Erythrina)  are  particularly 
favored  by  the  sun-birds. 

Whatever  other  advantages  may  come  from 
cross-pollination,  increased  variability  is  undoubt- 
edly one  result,  and  this  may  account  in  part  for  the 
rapid  evolution  of  new  forms  among  the  angio- 
sperms,  and  their  enormous  preponderance  at  the 
present  time. 

Fruits  of  Angiosperms. — The  fruits  of  the  angio- 
sperms  show  a  variety,  which  while  not  equal  to 
that  of  the  flowers,  nevertheless  is  very  great.  The 
lower  types  have  the  fruit  in  the  form  of  a  dry  cap- 
sule, which  opens  at  maturity  and  scatters  the  seed, 
or  sometimes  it  may  be  an  indehiscent  one-seeded 
fruit,  like  that  of  many  grasses,  or  the  buttercup. 
Such  fruits  have  often  to  depend  upon  chance  for 
their  distribution  of  seeds,  just  as  the  lower  floral 
types  are  dependent  upon  the  wind  for  transporting 
their  pollen.  With  the  evolution  of  plants,  however, 
many  modifications  of  the  fruit  arose  as  the  result 
of  which  the  distribution  of  the  seeds  was  facili- 
tated. This  distribution  may  be  in  some  cases  by 
means  of  water,  but  more  commonly  it  is  due  to  the 
wind,  or  the  agency  of  animals.  Of  fruits  adapted 
to  water  transport,  the  cocoanut  is  the  classic 
example.  The  gigantic  seed,  enclosed  in  its  thick 
water-proof  covering,  is  eminently  fitted  for  long 
immersion  without  suffering,  and  may  be  carried 
great  distances  by  the  ocean  currents.  Contrivances 


The  Angiosperms  181 

for  transportation  through  the  agency  of  the  wind 
are  extremely  numerous,  but  will  have  to  be  passed 
over  here.  As  well  as  assisting  in  the  pollina- 
tion of  flowers,  animals  are  very  important 
agents  in  transporting  the  fruits,  which  may  be  at- 
tractive to  them  as  food,  in  which  case  the 
enclosed  seeds  are  thrown  aside  and  scattered, 
or  are  swallowed  and  pass  through  the  body 
of  the  animal  undigested,  and  are  ejected  in  the 
excreta.  Birds  are  especially  important  in  this 
distribution  of  seeds,  owing  to  their  rapid  flight 
and  their  long  migrations,  and  doubtless  many 
widespread  species  of  plants  owe  their  distribution 
mainly  to  bird  agency.  Other  modifications  asso- 
ciated with  the  distribution  of  seeds  through  animal 
agencies  are  the  adhesive  organs  of  seeds  and  fruits 
by  which  they  cling  to  the  coats  of  animals  and  are 
carried  from  place  to  place. 

Angiosperms  Adapted  to  All  Conditions  of  Life. 
— The  many  special  modifications  of  the  angio- 
sperms,  adaptations  to  all  the  varied  conditions  of 
life,  can  only  be  touched  upon  here,  but  later  will 
be  considered  more  at  length.  Among  them  there 
are  plants  fitted  to  almost  every  possible  condi- 
tion under  which  plants  can  grow  at  all.  While 
reaching  their  most  luxuriant  development  in  the 
hothouse  conditions  of  the  equatorial  lowlands,  they 
can  also  grow  at  the  very  limits  of  vegetation  in  the 
polar  regions,  or  upon  alpine  summits ;  under  the 
fierce  sun  of  the  desert,  or  completely  submerged 


182  Plant  Life  and  Evolution 

in  the  ocean.  Struggling  for  light  in  the  fierce 
competition  in  the  tropical  zones,  climbing  plants 
of  many  types  have  been  evolved,  and  many  epi- 
phytes, or  air  plants,  may  be  seen  perched  almost 
at  the  very  tops  of  lofty  trees.  Some  species,  like 
the  giant  Rafflesia,  are  parasites  of  the  most  ex- 
treme type,  and  pass  nearly  their  whole  existence 
within  the  tissues  of  their  host,  exactly  as  a  fungus 
does,  and  like  the  fungus  they  expand  their  repro- 
ductive parts  in  the  air.  A  still  larger  number  are 
more  or  less  completely  saprophytic,  extracting  their 
nourishment  from  the  organic  debris  of  the  forests, 
much  as  the  toadstools  and  other  larger  fungi  do; 
but  it  may  be  stated  that  in  order  to  do  this  they 
seem  obliged  to  call  in  the  assistance  of  a  true 
fungus,  with  which  they  always  seem  to  be  asso- 
ciated. These  are  but  a  few  of  the  manifold 
adaptations  shown  by  this  protean  plant-type. 

SUMMARY 

While  the  early  history  of  the  angiosperms  is 
wrapped  in  obscurity,  the  evidence  at  hand  indicates 
that  the  first  angiosperms  probably  appeared  rather 
suddenly  towards  the  end  of  the  Mesozoic.  Whether 
they  arose  from  gymnosperms,  possibly  allied  to 
cycads  of  the  Mesozoic,  or  whether  they  were  de- 
rived more  directly  from  some  fern-like  ancestors, 
must  for  the  present  remain  unanswered.  The  re- 
markable uniformity  in  their  essential  structures, 


The  Angiosperms  183 

shown  by  all  of  the  existing  angiosperms,  makes 
it  almost  certain  that  they  are  all  derived  from  some 
common  stock,  or  at  any  rate  from  a  group  of  forms 
closely  related  to  each  other.  Once  established,  the 
angiospermous  type  showed  itself  to  be  peculiarly 
adaptable,  and  it  rapidly  assumed  a  predominant 
position.  Whence  arose  their  extraordinary  plas- 
ticity can  only  be  conjectured.  The  type  of  fruit, 
with  its  complete  protection  of  the  seed  until  its 
maturity,  may  have  been  one  of  the  important  fac- 
tors in  establishing  this  superiority  over  the  cycads 
with  their  exposed  seeds,  although  it  must  be  said 
that  in  the  cycads  the  growing  seed  is  generally 
more  or  less  protected  by  the  scales  of  the  cone.  But 
this  will  not  explain  the  extremely  plastic  plant  body 
which  contrasts  so  strongly  with  the  limitations  of 
the  plant-body  in  the  gymnosperms. 

It  is  possible  that  cross-pollination  among  the 
angiosperms  developed  very  early,  and  that  thus 
there  was  induced  a  greater  degree  of  variability 
resulting  in  the  appearance  of  many  modifications 
which  could  be  seized  upon  by  natural  selection, 
and  thus  tend  to  develop  new  types.  Whatever 
may  have  been  the  reason,  it  is  their  extraor- 
dinary adaptibility  that  is  at  the  bottom  of 
the  remarkable  success  attained  by  the  angiosperms. 
One  very  important  phase  of  this  is  the  utiliza- 
tion of  animals  for  distribution  of  pollen  and 
seeds.  This  is  not  absolutely  confined  to  the  angio- 
sperms, as  occasionally  the  spores  of  fungi  are 


184  Plant  Life  and  Evolution 

distributed  by  insects,  and  Welwitschia,  one  of  the 
gymnosperms,  but  possibly  allied  to  the  angio- 
sperms,  is  supposed  to  be  entomophilous.  Most  of 
the  plants  whose  organs  have  been  modified  with 
reference  to  animal  structures  are  angiosperms,  and 
the  extraordinary  variety  of  flowers  and  fruits  is 
doubtless  due  in  a  large  measure  to  such  adaptations. 

Which  is  the  older  of  the  two  main  divisions  of 
angiosperms  must  remain  for  the  present  in  doubt. 
The  geological  record  is  very  unsatisfactory  on 
this  point,  and  comparative  morphology  gives 
hardly  any  more  certain  answer.  It  is  possible,  at 
least,  that  the  divisions  into  monocotyledons  and 
dicotyledons  is  a  somewhat  artificial  one ;  and  it  may 
be  that  from  an  indifferent  primitive  stock,  angio- 
sperms in  all  essential  respects,  a  number  of  lines 
arose,  some  to  become  monocotyledons,  others  di- 
cotyledons. 

The  question  as  to  the  nature  of  the  primitive 
angiospermous  flower  is  also  not  at  all  satisfactorily 
settled.  While  some  of  the  diclinous  floral  types 
can  be  explained  as  reduced  from  hermaphrodite 
ones,  it  is  by  no  means  always  the  case,  and  we  be- 
lieve that  some,  at  least,  of  these  diclinous  types, 
are  really  primitive.  This  implies  that  before 
monocotyledons  and  dicotyledons  were  established 
as  such,  both  types  of  flowers  had  developed, 
and  these  in  the  further  course  of  evolution  were 
transferred  to  both  monocotyledonous  and  dicoty- 
ledonous families. 


The  Angiosperms  185 

Whatever  may  have  been  their  origin,  the  extraor- 
dinary fitness  of  these  plants  to  modern  conditions 
is  obvious,  and  they  have  taken  possession  of  the 
land-areas  of  the  whole  world  almost  to  the  com- 
plete exclusion  of  other  plant  types.  Only  under 
exceptionally  favorable  conditions  are  the  lower 
plant  types  able  to  hold  their  own  in  competition 
with  the  all-conquering  angiosperms. 


CHAPTER  VII 
ENVIRONMENT  AND  ADAPTATION 

WHAT  were  the  conditions  prevailing  upon 
the  earth  when  the  first  organisms  ap- 
peared, must  remain  purely  conjectural.  The  earliest 
forms  of  life  have  left  no  recognizable  traces,  and 
the  first  unmistakable  plant  remains  are  so  highly 
organized  as  to  make  it  certain  that  these  must  have 
been  preceded  by  a  long  series  of  simpler  forms. 
As  we  have  already  indicated  there  is  some  reason 
to  believe  that  the  bacteria  and  blue-green  algae 
more  nearly  approach  the  primordial  plants  than 
do  any  other  living  forms;  but  whether  or  not 
these  were  the  progenitors  of  the  higher  plants  is 
another  question.  The  resistance  of  many  of  these 
organisms  to  very  high  temperatures  and  other  con- 
ditions which  are  not  favorable  to  the  higher  plants, 
suggest  that  the  conditions  of  life  during  the  earliest 
history  of  the  plant  kingdom  were  different  from 
those  existing  at  present,  but  of  course  we  can 
only  guess  what  these  conditions  were. 

The  Higher  Plants  Derived  from  Algae. — A  study 
of  the  evolution  of  the  higher  plants  makes  it  al- 
most certain  that  these  are  descended  from  green 

186 


Environment  and  Adaptation        187 

algae,  probably  not  very  different  from  some  of  the 
existing  fresh-water  forms.  The  extraordinary 
persistence  of  the  motile  reproductive  cells,  found 
as  high  up  as  the  lowest  of  the  seed-plants,  indi- 
cates that  the  ancestors  of  the  modern  land  plants 
were  algae,  whose  pedigree  can  be  traced  back  to 
free-swimming  unicellular  plants,  resembling  some 
of  the  existing  Volvocales.  These  green  "  Monads  " 
may  be  assumed  to  have  abounded  in  the  seas  of 
the  earlier  geological  epochs.  It  has  been  assumed 
that  the  water  of  the  primordial  seas  was  fresh  or 
only  weakly  saline,  and  would,  therefore,  have  been 
adapted  to  the  existence  of  forms  like  the  modern 
green  algae,  which  at  present  are  mainly  fresh-water 
species.  With  the  increasing  salinity  of  the  ocean, 
many  of  these  more  primitive  green  algae  probably 
retreated  to  the  smaller  fresh-water  bodies,  where 
they  have  persisted,  perhaps  but  little  changed,  from 
the  remotest  times ;  while  in  the  more  saline  ocean 
water,  the  two  groups  of  typical  seaweeds,  the  red 
and  brown  algae,  have  developed  and  taken  the 
place  of  the  primitive  green  forms,  their  peculiar 
characters  becoming  more  and  more  pronounced 
with  the  increasing  salinity  and  other  changes  of 
their  environment. 

Uniform  Conditions  in  Fresh  Water. — Except 
for  differences  of  temperature  the  conditions  of  life 
in  fresh  water  are  very  uniform,  and  it  is  not  re- 
markable that  the  range  of  structure  in  the  fresh- 
water algae  is  relatively  slight.  The  most  primitive 


1 88  Plant  Life  and  Evolution 

of  the  unicellular  Volvocales  may  be  taken  to  repre- 
sent the  commencement  of  the  series  leading  up  to 
the  higher  plants.  The  earliest  plants  may  be  as- 
sumed to  have  been  motile  like  the  existing  Volvo- 
cales, but  the  power  of  motion  was  probably  early 
lost  in  the  vegetative  cells,  this  being  associated  with 
the  power  of  photosynthesis,  which  does  not  make 
it  necessary  for  the  plant  to  move  about  for  food. 
So  long  as  the  plant  is  completely  immersed  no 
special  organs  are  necessary  for  absorbing  the  water 
with  the  dissolved  food  constituents,  these  being 
taken  in  freely  at  all  points  of  the  surface.  More- 
over, such  submerged  plants  are  not  subject  to  loss 
of  water  through  evaporation,  and  therefore  the 
superficial  cells  do  not  need  to  develop  a  cuticle. 

MARINE  PLANTS 

The  relation  of  the  seaweeds  to  the  simpler  and 
probably  more  ancient  fresh-water  algse  is  largely 
a  matter  of  speculation.  The  true  brown  algae 
are  hardly  at  all  represented  in  fresh  water,  but 
there  are  a  number  of  fresh-water  organisms  which 
may  be  remotely  related  to  them.  The  development 
of  large  amounts  of  gelatinous  or  mucilaginous  tis- 
sues which  hold  the  water  with  great  tenacity,  and 
also  absorb  water  very  rapidly  when  the  plants  are 
wet  by  the  rising  tide,  has  already  been  referred  to. 
Some  of  the  large  kelps  may  be  exposed  to  the 
air  for  days  before  all  of  the  moisture  is  lost  from 


Environment  and  Adaptation        189 

their  tough  leathery  fronds.  This  power  of  re- 
taining water  is  exhibited  in  a  lesser  degree  by  some 
fresh-water  algae,  especially  those  forms  which  live 
in  the  temperate  regions  and  are  only  part  of  the 
time  in  the  water. 

Owing  to  their  perfect  adaptation  to  life  in  salt 
water,  the  brown  and  red  algse  have  found  few  com- 
petitors and  may  be  said  to  dominate  the  flora  of 
the  sea.  Whatever  may  be  the  reason,  salt  water 
seems  to  exercise  a  stimulus  which  induces  a  much 
more  luxuriant  vegetation,  and  as  we  have  seen  pro- 
duces greater  variation  than  does  fresh  water.  Sea- 
weeds growing  in  pure  sea  water  have  been  found 
to  be  more  robust  than  the  same  species  growing 
in  brackish  water,  for  instance  near  the  mouths  of 
streams.  The  common  rock- weed  (Fitcns  vesicu- 
losus)  and  one  of  the  red  algae,  a  species  of  Poly- 
siphonia,  are  examples. 

Marine  Algae. — The  red  algae  are  not  so  ex- 
clusively marine  in  their  habit,  and  the  lower  mem- 
bers of  the  class  show  sufficient  points  of  resem- 
blance to  the  green  algae  to  make  it  possible  that 
they  may  be  offshoots  of  the  latter.  The  conditions 
of  life  in  the  sea  are  very  different  from  those  in 
fresh  water,  perhaps  the  greatest  difference  being 
the  marked  salinity,  and  consequently  greater  density 
of  sea  water — a  condition  which  has  involved  great 
changes  in  the  structures  of  marine  plants.  The 
conditions  in  the  sea  are  evidently  conducive  to  great 
variability,  and  we  find  the  seaweeds  reaching  a  size 


190  Plant  Life  and  Evolution 

and  complexity  with  which  none  of  the  fresh-water 
algae  can  compare.  This  culminates  in  the  giant 
kelps,  whose  great  leafy  shoots  may  be  hundreds  of 
feet  in  length  and  in  their  form  suggest  the  higher 
land  plants.  That  the  increase  of  salinity  seems  to 
induce  variation  has  been  noted  repeatedly.  It  has 
been  observed  that  in  adjacent  areas,  differing 
merely  in  their  salinity,  the  less  saline  water  is  very 
much  poorer  in  the  number  of  species  than  the  more 
saline  water.  While  the  brown  and  red  algae  pre- 
dominate in  the  sea,  there  are  many  green  algse 
found  there  also,  and  the  latter  are  much  less  sensi- 
tive to  changes  in  salinity  of  water  than  are  the 
more  highly  specialized  brown  and  red  seaweeds, 
the  latter  of  which  are  often  quickly  killed  by  slight 
changes  in  temperature  and  salinity.  A  good  many 
species  of  both  brown  and  red  algse  show  a  cer- 
tain amount  of  adaptability,  and  may  adjust  them- 
selves to  slightly  saline,  and  even  brackish  water,  as, 
for  instance,  near  the  mouths  of  rivers  flowing  into 
the  sea. 

Marine  Life  Checks  Sexual  Reproduction. — It 
may  be  safely  assumed  that  the  green  seaweeds  are 
probably  immigrants  from  fresh  water,  which  have 
become  modified  more  or  less  by  their  changed  en- 
vironment. A  striking  peculiarity  of  all  of  the 
green  seaweeds  is  the  primitive  condition  of  their 
reproductive  parts,  although  many  of  them  are 
plants  of  considerable  complexity.  No  green  sea- 
weeds are  known  in  which  the  gametes  are  perfectly 


Environment  and  Adaptation        191 

differentiated  into  eggs  and  spermatozoids.  It  may 
be  that  life  in  salt  water  may  tend  to  check  the 
evolution  of  the  sexual  system.  It  is  true  that  in 
one  group  of  brown  algae,  the  Fucacese,  sexuality 


FIG.  20 

The  Sea-palm  (Postelsia),  an  alga  adapted  to  life  in  the 
surf;  the  long  flexible  stem  is  firmly  anchored  to  the  rocks 
by  a  powerful  holdfast,  a  root-like  grappling  organ. 


is  well  developed,  but  in  a  very  much  larger  num- 
ber the  sexual  elements  are  very  simple,  being 
motile  gametes,  or  the  plant  may  be  entirely  sexless. 
In  the  largest  of  all  of  the  brown  algae,  the  giant 


192  Plant  Life  and  Evolution 

kelps,  no  sexual  reproduction  has  been  demon- 
strated. 

It  is  pretty  clear  that  the  lack  of  resting  spores 
in  the  marine  algse  is  correlated  with  their  absolute 
freedom  from  danger  of  drying  up.  It  may  be  that 
the  advantages  of  the  numerous  quickly  germinating 
reproductive  cells,  such  as  the  zoospores  and  iso- 
gametes  of  most  of  the  brown  and  green  seaweeds, 
or  the  spores  of  the  red  algae,  have  been  so  great 
that  they  have  remained  in  their  present  state  of 
development  as  the  result  of  natural  selection. 

Surf  Algae.  — Certain  very  obvious  adaptations  to 
a  marine  environment  are  the  tough  and  flexible 
tissues  found  in  the  larger  kelps  and  red  seaweeds 
which  are  exposed  to  violent  surf.  This  is  very 
beautifully  shown  in  the  great  kelps  that  abound 
along  the  rocky  coasts  of  Pacific  North  America. 
Some  of  them,  like  the  sea-palm  (Postelsia)  (Fig. 
20),  cling  to  the  most  exposed  rocks,  where  they 
are  constantly  battered  by  the  full  force  of  the  heavy 
waves  that  dash  against  the  shore.  These  kelps,  with 
their  elastic  leathery  fronds  and  powerful  holdfasts, 
grip  the  rocks  securely  and  withstand  uninjured  the 
heaviest  pounding  of  the  surf. 

TERRESTRIAL  PLANTS 

The  conditions  of  life  in  water  are  much  less 
variable  than  on  land.  Temperature  changes  are  less 
extreme  and  rapid,  and  of  course  the  amount  of  wa- 


Environment  and  Adaptation        193 

ter  supplied  to  the  plant  is  constant,  and  provisions 
for  the  conduction  of  water  and  for  its  conserva- 
tion are  unnecessary;  hence  the  absence  of  these 
in  such  submersed  aquatics  as  most  of  the  algae. 
However,  where  the  plant  reaches  a  great  size,  as 
in  the  kelps,  conducting  tissues  for  the  transport 
of  assimilated  material  may  be  very  perfectly 
developed.  While  variations  of  light  to  which  algae 
are  exposed  are  somewhat  less  extreme  than  is  the 
case  in  land  plants,  it  is  evident  that  the  question 
of  light  has  been  one  of  the  most  important  factors 
in  the  modifications  of  the  algal  types,  since  the 
varying  depth  of  the  water,  as  well  as  the  shade 
of  the  rocks  and  larger  algae,  must  cause  great  dif- 
ferences in  the  intensity  of  the  light,  with  a  corre- 
sponding variation  in  the  plants  adapted  to  these 
lights  of  different  intensity. 

As  we  have  endeavored  to  show,  the  first  land 
plants  probably  arose  from  forms  allied  to  some 
of  the  existing  fresh-water  algae,  which  became 
adapted  to  life  on  land  by  the  development  of  roots 
for  water  absorption,  and  more  or  less  perfect 
protection  of  the  exposed  tissues  against  undue 
loss  of  water.  This  is  secured  either  by  the  de- 
velopment of  mucilaginous  or  gelatinous  envelopes, 
or  by  the  cuticularization  of  the  exposed  cell  walls. 
This  primitive  type  of  land  plant  probably  cul- 
minated in  the  higher  mosses,  but  it  never  became 
quite  perfectly  adjusted  to  terrestrial  conditions, 
since  the  simple  hair-like  roots  could  only  suffice 


194  Plant  Life  and  Evolution 

for  a  plant  of  moderate  size,  and  the  mechanical, 
or  supporting  tissues,  which  enable  the  terrestrial 
plants  to  overcome  the  force  of  gravity,  are  indif- 
ferently developed  in  the  mosses.  It  has  also  been 
pointed  out  that,  having  exhausted  the  possibilities 
of  an  aquatic  gametophyte,  after  its  translation  to 
land,  nature  seems  to  have  taken  up  the  neutral 
generation  or  sporophyte  as  a  more  promising  sub- 
ject for  further  experiments  in  the  development  of 
a  truly  terrestrial  plant  type.  The  sporophyte,  be- 
ing originally  an  adaptation  to  terrestrial  condi- 
tions, seems  to  have  a  much  greater  potentiality  for 
development  as  a  land  plant,  and  once  thoroughly 
established  as  such,  superseded  the  algae  and  mosses 
as  the  prevailing  type  of  land  vegetation.  The 
ferns  or  Pteridophytes  are  the  first  of  these  typical 
terrestrial  plants.  Their  preeminence  is  due  to  the 
development  of  true  roots  capable  of  indefinite 
growth  to  correspond  to  the  great  development  of 
the  rest  of  the  plant-body,  which  in  these  plants 
assumes  a  size  and  variety  far  surpassing  anything 
attained  by  the  lower  plants.  The  sporophyte  has 
shown  itself  to  be  extraordinarily  adaptable,  and  has 
been  able  to  establish  itself  under  very  different  con- 
ditions of  heat,  light,  and  moisture.  The  elaborate 
root  system,  with  the  development  of  very  perfect 
water  conducting  tissues,  provides  for  rapid  absorp- 
tion and  transport  of  water  within  the  plant,  and 
the  outer  tissues  are  effectively  protected  against 
undue  loss  of  water  by  transpiration. 


Environment  and  Adaptation        195 

On  the  basis  of  their  relation  to  water,  three 
categories  of  plants  are  recognized :  Hydrophytes 
or  aquatics,  Mesophytes  or  plants  in  which  there  is 
a  normal  supply  of  water  but  which  are  not  true 
aquatics,  and  Xerophytes  in  which  the  need  of  con- 
servation of  water  is  more  or  less  acute.  Of  course 
these  groups  are  not  absolutely  separate  from  each 
other,  and  may  be  further  subdivided. 


AQUATICS 

Aside  from  the  algae,  nearly  all  of  which  are 
true  aquatics,  there  are  r».  good  many  flowering  plants 
and  a  few  mosses  and  ferns,  which  are  also  to  a 
greater  or  less  degree  genuine  water  plants.  How 
far  these  forms  are  secondary,  that  is,  are  derived 
from  originally  terrestrial  types,  is  not  always  easy 
to  decide ;  but  in  many  cases  it  is  perfectly  clear  that 
they  are  modified  descendants  of  terrestrial  forms. 
These  aquatics  may  be  completely  immersed,  as  in 
some  of  the  pond-weeds  and  some  of  the  sea-plants, 
like  the  eel-grass  (Zostera)  ;  or  they  may  be  float- 
ing plants,  like  the  little  water- fern  (Azolla),  or 
the  duck- weed  (Lemna),  etc.,  or  they  may  be 
rooted  below  the  surface  of  the  water  with  floating 
leaves  and  flowers,  like  the  water-lilies,  or  finally 
they  may  stand  above  the  surface  like  the  reeds, 
cattail-rushes,  etc. 

Land  and  Water  Plants  Compared. — Compared 
with  the  related  land  plants,  these  aquatics  show 


196  Plant  Life  and  Evolution 

various  modifications.  Where  the  plant  is  com- 
pletely submersed,  the  exterior  tissues  are  quite  des- 
titute of  a  cuticle,  and  the  stomata  are  entirely  ab- 
sent; but  if  any  parts  emerge  above  the  water,  such 
for  instance  as  the  upper  surface  of  the  water-lily 
leaf  or  the  aerial  leaves  of  the  arrow-head,  the  epi- 
dermis shows  the  usual  cuticle  and  stomata.  Sub- 
mersed leaves  are  generally  either  narrow  or 
finely  dissected,  and  the  contrast  between  the  sub- 
mersed and  aerial  leaves  of  the  same  species  is 
often  very  striking.  Thus  in  the  water-shield 
(Cabomba),  and  some  species  of  water  crowfoot, 
the  aerial  leaves  are  quite  entire,  or  only  slightly 
lobed,  while  the  submersed  leaves  are  finely  divided 
into  very  narrow  segments.  How  far  the  peculiar 
form  of  submersed  leaves  is  directly  due  to  the 
physical  properties  of  the  surrounding  medium,  and 
how  much  is  to  be  attributed  to  adaptation  to  light 
and  food  conditions,  has  not  been  satisfactorily 
demonstrated.  The  more  direct  exposure  to  light 
and  to  the  action  of  CO2  and  free  oxygen  dissolved 
in  the  water,  are  probably  important  factors  con- 
cerned with  the  form  of  these  submersed  leaves. 

Comparatively  few  woody  plants  are  aquatics,  and 
where  they  have  roots  completely  submerged  they 
may  show  some  interesting  modifications,  usually 
associated  with  the  aeration  of  the  roots.  The  curi- 
ous growths  from  the  roots  of  the  southern  cypress, 
known  as  "  cypress-knees,"  and  the  aerial  roots  of 
the  mangrove,  are  undoubtedly  aerating  organs. 


Environment  and  Adaptation         197 

The  hydrophytes  are  sometimes  characterized  by 
the  poor  development  of  roots,  which  may  actually 
be  absent  ia  some  of  them.  Most  of  the  hydro- 
phytes are  herbaceous  plants,  and  the  stems  and  leaf 
stalks  are  provided  with  very  large  air  spaces. 

MESOPHYTES 

Where  plants  are  provided  with  an  adequate  but 
not  excessive  amount  of  water,  they  develop  a 
perfect  root-system,  and  an  ample  expanse  of  green 
tissue,  either  in  the  form  of  a  flat  thallus,  or,  in  the 
higher  plants,  of  leaves  of  various  kinds.  The  size 
of  the  leaf  is  to  a  certain  extent  dependent  upon  the 
intensity  of  light  and  upon  the  amount  of  moisture, 
the  two  often  being  in  inverse  ratio.  Other  things 
being  equal,  transpiration  is  less  active  in  the  shade 
than  in  the  full  light,  and  shade  plants  normally 
exhibit  a  much  larger  leaf  surface  than  those  ex- 
posed to  full  sunlight.  The  difference  is  very  evi- 
dent in  plants  of  the  same  species,  or  even  in  the 
same  individual,  and  can  be  readily  enough  demon- 
strated. If  the  light  is  completely  excluded,  or  is 
too  weak  for  photosynthesis,  there  usually  is  a  de- 
generation of  the  leaf  lamina,  which  may  be  almost 
completely  suppressed,  a  fact  familiarly  demon- 
strated by  the  blanched  and  shrunken  leaves  of  a 
plant  sprouted  in  the  dark. 

Types  of  Leaves  in  Mesophytes. — The  increase 
in  the  extent  of  the  leaf  surface  may  be  effected  in 


198  Plant  Life  and  Evolution 

various  ways.  Some  plants  develop  very  many  small 
leaves,  others  a  few  very  large  ones,  and  the  latter 
may  be  entire  as  in  the  banana  and  many  arums, 
or  it  may  be  very  much  divided  as  in  some  of  the 
tree-ferns.  Mesophytes  predominate  in  the  moister 
temperate  regions  and  in  the  shaded  forests  of  the 
tropics.  With  increasing  moisture  they  approxi- 
mate the  hydrophytic  type,  and  as  the  moisture  de- 
creases they  assume  more  xerophytic  characters. 

XEROPHYTES 

The  term  "  xerophyte  "  has  been  applied  to  those 
plants  which  exhibit  more  or  less  evident  characters 
adapting  them  to  growth  with  a  limited  water 
supply.  As  the  amount  of  water  is  diminished  in  the 
normally  mesophytic  plant,  it  becomes  dwarfed  and 
the  leaves  are  very  much  smaller,  and  at  the  same 
time  there  is  a  thickening  of  the  leaves,  and  often 
greater  hairiness.  With  the  reduced  leaf  surface 
there  is  naturally  a  correspondingly  diminished 
transpiration  of  water. 

Xerophytes  Not  Always  Confined  to  Dry  Re- 
gions.— While  most  mesophytes  are  able  to  adapt 
themselves  to  a  greater  or  less  reduction  in  the  water 
supply,  there  are  many  plants  which  normally  grow 
in  regions  where  they  can  receive  only  a  very  lim- 
ited amount  of  water,  and  it  is  among  these  natural 
xerophytes  that  the  most  remarkable  adaptations 
for  economizing  water  are  found.  Xerophytes  are 


Environment  and  Adaptation        199 

by  no  means  confined  to  very  dry  regions,  but  may 
be  found  almost  everywhere,  even  in  regions  of 
heavy  rainfall.  The  character  of  the  soil  and  the 
exposure  may  be  such  as  to  allow  most  of  the  water 
that  falls  to  escape,  and  plants  growing  under  such 
conditions  must  provide  for  this.  Thus  a  plant 
growing  on  a  steep  gravelly  hillside,  or  in  the 
crevice  of  a  rocky  cliff,  can  use  only  a  very  small 
part  of  the  rain  that  falls  upon  it,  and  consequently 
such  plants  will  show  a  more  or  less  pronounced 
xerophytic  habit. 

Desert  Plants. — Of  course  it  is  in  the  more  arid 
parts  of  the  world  that  the  xerophytes  abound, 
and  it  is  these  desert  plants  that  offer  the  most 
striking  examples  of  xerophytic  adaptation.  The 
simplest  method  of  checking  the  loss  of  water  is  by 
reducing  the  number  and  size  of  the  leaves,  and  in- 
creasing the  thickness  of  the  epidermis.  A  plant 
growing  on  the  dry  hillside,  contrasted  with  the 
same  species  in  the  moist  valley  below,  will  show  this 
very  clearly.  In  the  more  pronounced  xerophytes 
the  leaves  may  be  entirely  lost,  as  in  the  Spanish 
broom  or  in  many  cacti  and  some  euphorbias.  In 
such  xerophytes  the  photosynthetic  function  is  taken 
over  by  the  superficial  tissues  of  the  stem. 

This  desert  vegetation  is  very  strikingly  devel- 
oped in  the  hot,  arid  regions  of  Northern  Mexico, 
and  the  adjacent  deserts  of  Southern  California  and 
Arizona.  Among  the  most  striking  xerophytes  of 
this  region  are  the  innumerable  cacti,  some  like  the 


2OO  Plant  Life  and  Evolution 

giant  cactus,  or  Suwarro  (Cereus  gigantens},  hav- 
ing almost  tree-like  proportions,  and  showing  a  max- 
imum reduction  of  the  evaporation  surface,  and 
extraordinary  capacity  for  water  storage.  The  ex- 
tensively branched  roots  run  close  to  the  surface  of 
the  ground,  where  they  quickly  absorb  the  rain  and 
convey  it  to  the  great  pillar-shaped  stem,  where  it 
is  stored  away  deep  in  the  sappy  tissues  of  the  pith. 

The  century  plant  (Agave),  unlike  the  cacti,  has 
its  leaves  highly  developed,  but  these  are  very  thick 
and  fleshy,  and  protected  by  an  impervious  covering 
so  that  they  are  quite  as  efficient  water-storage  or- 
gans as  the  fleshy,  leafless  stems  of  the  cacti. 

Another  remarkable  plant  (Fouquiera)  of  this 
same  region  is  familiar  to  every  observant  traveler 
through  Southern  Arizona.  This  curious  shrub, 
known  locally  as  "  Fish-pole  Cactus,"  or  "  Oco- 
tilla,"  is  a  bush  consisting  of  a  bundle  of  slender 
stems,  sometimes  tipped  with  a  cluster  of  scarlet 
flowers,  nearly  or  quite  unbranched,  and  usually 
quite  bare  of  leaves.  Periodically,  however,  at  the 
seasons  when  the  brief  showers  of  midsummer  or 
midwinter  fall,  the  bare  stems  clothe  themselves 
with  delicate  little  leaves,  quite  out  of  keeping  with 
the  desert  environment,  and  these  quickly  wither 
with  the  cessation  of  the  rains. 

Sometimes  the  leaf  becomes  reduced  so  that  the 
normal  blade  disappears,  and  a  flattened  leaf  stalk 
takes  its  place.  These  "  phyllodia  "  are  especially 
perfect  in  some  of  the  Australian  acacias,  often 


Environment  and  Adaptation        201 

grown  for  ornament,  and  their  real  nature  can  be 
seen  by  tracing  the  development  of  the  leaves  in 
the  seedling,  which  at  first  always  show  a  feathery 
lamina,  which  is  gradually  reduced  in  the  later 
leaves,  until  it  quite  disappears,  and  nothing  is  left 
but  a  flattened  leaf  stalk.  When  these  normally 
xerophytic  plants  are  abundantly  watered,  it  is  quite 
common  to  find  the  leaves  reverting  to  the  feathery 
form  on  the  more  vigorous  young  shoots.  In  other 
cases  like  the  prickly  pear  and  the  greenhouse 
"  smilax  "  the  leaf-like  organs  are  really  flattened 
branches. 

Water  Storage. — The  storage  of  water  is  also  an 
important  function  in  xerophytic  plants,  and  there 
are  many  types  of  storage  organs.  The  root  sys- 
tem is  also  modified  with  reference  to  the  water 
supply  and  to  the  character  of  the  aerial  parts.  In 
desert  plants  the  roots  may  be  very  long  and  capa- 
ble of  reaching  down  to  the  deep-seated  layer  of 
water  in  the  soil,  or  as  has  recently  been  shown  by 
Cannon  in  his  studies  on  the  roots  of  desert  plants, 
the  root  system  may  be  very  shallow,  spreading  ex- 
tensively near  the  surface  of  the  ground,  where 
advantage  may  be  taken  of  brief  showers  which 
wet  only  the  superficial  layers  of  the  soil,  and 
quickly  evaporate.  The  water  is  promptly  absorbed 
and  conveyed  to  the  aerial  parts  of  the  plant  and 
there  stored  away  for  future  use.  The  great  cacti 
of  the  hot,  arid  regions  of  the  Southwest  are  very 
striking  examples  of  this  xerophytic  type. 


2O2  Plant  Life  and  Evolution 

Many  xerophytes,  like  the  cacti  and  century-plant, 
have  fleshy  stems  or  leaves  with  impervious  outer 
tissues,  which  prevent  loss  of  water,  while  the 
inner  tissues  are  often  mucilaginous  and  very  re- 
tentive of  the  moisture  which  is  stored  up  in  great 
amounts.  Such  fleshy  plants  can  be  uprooted  and 
exposed  for  weeks  to  the  hot  sun  before  all  their 
moisture  is  lost. 

Bulbous  Plants. — Another  xerophytic  type  is  seen 
in  the  bulbous  and  tuberous  plants,  which  are  char- 
acteristic of  many  semi-arid  regions  like  California, 
the  shores  of  the  Mediterranean,  and  the  Cape  re- 
gion of  South  Africa.  These  bulbs  or  tubers  may 
be  exposed  to  drying  up  without  losing  their  vital- 
ity, and  will  be  found  to  retain  a  large  amount  of 
water  for  a  long  period.  When  proper  conditions 
arise  for  their  growth,  the  leaves  and  flowers  are 
rapidly  developed  at  the  expense  of  the  moisture 
and  food  stored  up  in  the  bulb,  and  after  the  seeds 
have  matured,  they  wither  away,  leaving  only  the 
subterranean  portions  alive.  Many  familiar  garden 
flowers  are  of  this  type,  most  of  them  coming  from 
regions  with  a  more  or  less  pronounced  dry  season. 
California  is  very  rich  in  bulbous  plants,  and  many 
of  these,  like  the  beautiful  Mariposa-lily  (Calo- 
chortus),  are  among  the  most  charming  of  our  wild 
flowers. 

Halophytes. — Resembling  in  many  respects  the 
true  xerophytes,  are  the  Halophytes,  or  salt-marsh 
plants.  Although  they  grow  where  there  seems  to 


Environment  and  Adaptation        203 

be  an  abundance  of  moisture,  these  plants  take  up 
a  relatively  small  amount,  as  the  excess  of  salt  dis- 
solved in  the  water  is  not  favorable  for  their 
growth.  Hence  these  plants  show  the  fleshy  habit 
characteristic  of  xerophytes.  Plants  growing  along 
the  seashore  often  show  this  same  fleshy  texture. 
Examples  of  these  strand  plants  are  the  ice-plants 
(Mesembryanthemum),  sand  verbena  (Abronia), 
and  sea-rocket  (Cakile). 

LIGHT 

Modifications  Associated  with  Photosynthesis — 

As  photosynthesis  is  the  most  important  of  the  nu- 
tritive processes  of  the  plant,  it  is  not  surprising 
that  some  of  the  most  striking  modifications  of  the 
plant-body  are  obviously  associated  with  chloro- 
phyll work.  In  the  cells  of  all  of  the  algae  there 
are  present  definite  organized  structures,  chromato- 
phores,  which  are  the  essential  photosynthetic  or- 
gans, and  the  form  and  position  of  these  is  to  a 
great  extent  correlated  with  the  direction  and 
the  intensity  of  the  light  rays.  In  the  lowest  forms 
there  is  usually  a  single  large  chromatophore  which 
may  assume  a  very  complicated  form,  as  in  some  of 
the  pond-scums  (Spirogyra)  or  the  desmids;  or 
there  may  be  two  or  more  large  chromatophores, 
which  are  often  more  or  less  divided,  possibly  a  con- 
trivance for  increasing  their  efficiency.  Only  a 
relatively  small  number  of  the  green  algae,  e.g., 


204  Plant  Life  and  Evolution 

Siphoneae  and  Characeae,  have  many  small  chromato- 
phores  like  those  of  the  higher  plants. 

The  prevalence  of  the  small  chromatophores  in 
the  more  highly  specialized  red  and  brown  algae,  as 
well  as  in  the  higher  land  plants,  would  indicate  that 
the  numerous  small  chromatophores  are  probably 
more  efficient  than  the  single  large  chromatophore 
of  the  lower  types.  Aside  from  the  modifications  of 
the  individual  chromatophores,  the  character  and  ar- 
rangement of  the  cells  containing  them  may  safely 
be  assumed  to  be  related  to  light  exposure.  In  the 
larger  and  more  massive  seaweeds,  the  chromato- 
phores are  mainly  developed  in  the  superficial  cells, 
where  they  are  best  exposed  to  the  light,  and  in 
the  more  delicate  algae  the  assimilative  cells  may  be 
spread  out  in  thin  leaf-like  plates,  exposing  a  large 
area  to  the  light,  and  sometimes  the  same  result  is 
obtained  by  the  development  of  dense  tufts  of  fine 
branches  composed  of  single  rows  of  small  cells, 
which  are  thus  exposed  on  all  sides. 

Among  the  red  and  brown  seaweeds,  secondary 
pigments  are  developed,  but  there  has  been  much  dis- 
cussion as  to  their  composition  and  as  to  the  role 
they  play  in  the  plants'  economy.  The  brown  pig- 
ments of  the  kelps  are  pretty  generally  recog- 
nized as  protective,  screening  the  chlorophyll  from 
too  strong  illumination.  The  brown  algae  very  com- 
monly grow  close  to  the  surface  of  the  water  or 
actually  floating  upon  it,  or  they  may  be  completely 
exposed  by  the  ebbing  tide.  Some  of  them  grow 


Environment  and  Adaptation        205 

in  deeper  water,  but  it  has  been  suggested  that 
those  living  in  deep  water  represent  a  secondary 
adaptation  which  has  not  caused  the  loss  of  the 
pigment,  which,  however,  is  no  longer  essential. 

The  case  of  the  red  algae  seems  to  be  somewhat 
different.  They  are,  as  a  rule,  shade-loving  plants, 
and  grow  either  in  deep  water  or  in  the  shade  of 
other  large  algae  or  of  rocks.  It  has  been  assumed 
that  the  red  pigments  of  these  forms  enable  them 
to  absorb  certain  light  rays  which  they  otherwise 
could  not  utilize.  How  far  this  is  connected  with 
their  living  in  deep  water,  which  absorbs  much  of 
the  red  and  yellow  rays  which  are  usually  essential 
to  photosynthesis  in  the  green  plants,  has  not  been 
satisfactorily  demonstrated. 

The  influence  of  light  in  affecting  the  form  of 
the  higher  plants  may  be  readily  shown  by  experi- 
ment. The  difference  in  habit  between  plants 
grown  in  dense  shade  and  the  bright  sunshine  is 
very  marked,  and  while  other  factors  than  light  are 
undoubtedly  concerned,  the  light  relation  is  one  of 
the  most  potent  factors  in  the  change  of  form. 
The  influence  of  light  in  determining  the  direction 
of  growth  of  plants  is  familiar  to  every  one,  most 
plants  growing  towards  the  light,  and  in  the  lower 
plants,  as  well  as  the  higher,  this  can  be  shown.  In 
the  flat  gametophyte  of  the  ferns  and  liverworts, 
the  direction  of  the  light  striking  it  determines  which 
is  to  be  the  upper  and  which  is  to  be  the  lower  side, 
and  it  has  been  recently  shown  by  Peirce  that  in 


206  Plant  Life  and  Evolution 

some  liverworts  the  dorsiventral  character  of  the 
thallus  can  be  inhibited  by  subjecting  all  parts  of 
the  developing  plant  to  equal  illumination. 

Relation  of  Photosynthetic  Organs  to  Light 

In  a  general  way  the  development  of  leaves,  or  the 
corresponding  photosynthetic  organs,  is  directly  as- 
sociated with  the  intensity  of  light,  which  up  to  a 
certain  optimum  is  more  and  more  efficient  as  the 
intensity  increases.  However,  many  plants  are  ex- 
posed to  an  intensity  of  light,  and  with  it  usually 
a  degree  of  heat,  which  is  in  excess  of  the  optimum, 
and,  therefore,  these  plants  have  developed  devices 
for  protecting  the  delicate  assimilating  tissues  from 
injury  which  might  result  from  excessive  illumina- 
tion. The  leaves  may  be  covered  with  a  very  thick 
epidermis,  which  is  sometimes  supplemented  by  a 
sub-epidermal  tissue  of  such  character  as  to  inter- 
cept much  of  the  light  and  heat;  or  there  may  be 
special  pigments  developed,  or  the  surface  of  the 
leaf  may  be  covered  with  masses  of  hairs  or  scales 
which  cover  the  exposed  surfaces  with  a  gray  or 
white  film. 

In  many  tropical  trees,  like  the  mango,  the 
young  leaves  are  limp,  and  hang  vertically,  while 
they  are  colored  pink  or  purple,  both  their  position 
and  the  development  of  the  special  pigments  being 
supposed  to  be  methods  of  neutralizing  the  effect 
of  the  powerful  sun's  rays  upon  the  delicate  as- 
similative tissues  of  the  young  leaves.  It  is  possi- 
ble that  the  deep  red  or  purple  color  of  the  young 


Environment  and  Adaptation        207 

leaves  of  tea-roses  may  have  somewhat  the  same 
function.  The  vertical  position  of  the  leaves  of 
Eucalyptus  and  of  the  Calif ornian  Manzanita,  and 
the  vertically  placed  phyllodes  of  the  acacias  al- 
ready referred  to,  are  also  supposed  to  be  pro- 
tective devices  against  too  powerful  illumination. 
It  is  not  easy  to  distinguish  between  modifications 
associated  directly  with  excess  of  light,  and  those 
concerned  merely  with  checking  transpiration,  which 
is  increased  by  both  light  and  high  temperature. 

Climbing  Plants. — As  light  is  the  all-essential 
factor  for  the  growth  of  green  plants,  it  is  not 
strange  that  the  struggle  for  light  in  the  teeming 
vegetation  of  the  tropics  has  resulted  in  many 
adaptations.  This  explains  the  various  types  of 
climbing  plants,  which,  although  sparingly  repre- 
sented in  the  temperate  regions,  must  be  seen  in  the 
damp  tropical  jungles  to  appreciate  their  full  possi- 
bilities. These  climbing  plants,  either  by  twining 
their  stems  about  others,  or  by  lifting  themselves  up 
by  tendrils  of  various  kinds,  may  climb  to  the  tops 
of  the  tallest  trees,  or  stretch  from  one  tree  to  an- 
other, often  completely  smothering  the  lower 
growths  over  which  they  spread  themselves.  Thus 
lifted  above  the  lower  vegetation,  they  expand  their 
leaves  and  flowers  in  the  full  sunshine  far  aloft. 

Epiphytes. — Another  type  of  adjustment  to  light 
is  seen  in  the  Epiphytes,  or  air  plants,  which  also 
are  best  developed  in  the  tropical  forests.  In  tem- 
perate climates  most  of  the  epiphytes  are  humble 


208  Plant  Life  and  Evolution 

plants  like  the  lichens  and  mosses,  but  under  more 
favorable  conditions  many  flowering  plants  and 
ferns  are  found  among  these  epiphytes.  In  the  wet 
tropical  forest,  the  trunks  and  branches  of  the  trees, 
and  even  the  surfaces  of  the  leaves,  may  be  covered 
with  a  tangle  of  liverworts,  ferns,  orchids,  and  even 
shrubs,  like  some  of  the  rhododendrons  and  vac- 
ciniums.  As  these  epiphytes  are  largely  dependent 
upon  the  atmospheric  moisture  for  their  water  sup- 
ply, they  are  often  more  or  less  xerophytic  in  habit, 
having  small  thick  leaves,  or  developing  special 
water-storage  organs,  like  the  "  pseudo-bulbs  "  of 
many  orchids,  and  the  water-storing  scales  of  the 
Spanish  moss  and  other  bromeliads.  A  good  many 
of  the  epiphytes,  especially  ferns,  collect  between 
their  closely  set  leaf  bases  masses  of  humus,  which 
serve  both  to  hold  moisture  and  to  provide  nourish- 
ment for  the  roots  which  ramify  through  the  humus, 
and  absorb  nourishment  from  it. 

Light  Not  Always  Necessary. — While  light  is  a 
necessary  factor  for  the  growth  of  all  green  plants, 
it  is  not  essential  for  the  existence  of  many  forms 
without  chlorophyll.  Thus  many  bacteria  normally 
live  in  complete  darkness,  and  certain  of  the  organs 
of  green  plants,  especially  the  subterranean  parts, 
develop  in  darkness. 

Light  exercises  a  powerful  effect  upon  the  devel- 
opment of  many  organs  which  are  not  connected 
with  photosynthesis.  The  fruiting  bodies  of  some 
fungi  are  not  perfectly  developed  except  in  light, 


Environment  and  Adaptation        209 

and  the  degree  of  light  often  exercises  a  strong 
influence  upon  the  production  of  the  reproductive 
organs  of  many  algae.  Many  flowering  plants,  also, 
growing  with  insufficient  light,  develop  few  or  no 
flowers. 

Nature  of  the  Light  Stimulus. — The  nature  of 
the  light  stimulus  is  very  obscure,  and  it  is  not  likely 
that  it  is  always  the  same.  In  some  cases,  e.g., 
where  certain  green  spores  refuse  to  germinate  in 
darkness — it  is  quite  probable  that  this  is  on  account 
of  the  failure  to  develop  certain  products  of  photo- 
synthesis before  germination  can  begin,  and  this  is 
made  the  more  probable,  as  sometimes  by  supplying 
sugar,  which  might  very  well  replace  some  of  the 
products  of  photosynthesis,  moss-spores  may  be 
made  to  germinate  in  darkness.  The  stimulus  of 
light  is  not,  however,  indispensable  in  all  cases,  as 
many  spores  normally  germinate  in  darkness,  and 
apparently  can  develop  the  necessary  stimulus  for 
growth  without  the  aid  of  light.  The  direct  effect  of 
light  upon  the  rate  of  growth  is  usually  a  retarda- 
tion. Plants  growing  in  darkness  become  exception- 
ally elongated,  and  there  is  also  a  difference  in  the 
size  of  the  leaves  growing  in  shade  and  in  full  light. 
How  far  the  larger  size  of  the  shade  leaf  is  the 
direct  effect  of  the  action  of  the  diminished  light, 
and  how  far  it  is  only  a  correlation,  bound  up  with 
the  necessity  for  a  greater  exposure  of  green  tissue 
owing  to  the  diminution  of  photosynthesis,  it  would 
be  hard  to  say. 


2io  Plant  Life  and  Evolution 

Excessive  Light. — The  effect  of  too  strong  light 
is  injurious  and  may  result  in  the  destruction  of  the 
delicate  tissues  exposed  to  it.  Hence  arises  the  ne- 
cessity in  many  plants  for  protection  against  exces- 
sive illumination,  and  as  this  is  usually  associated 
with  high  temperature,  and  consequently  rapid 
transpiration,  it  is  not  always  easy  to  determine  how 
far  certain  structures  are  connected  with  excessive 
light,  and  how  far  with  protection  against  heat  and 
loss  of  water.  Thus  the  thickened  cuticle  of  the 
leaves,  and  the  dense  covering  of  hair  often  found 
in  many  plants  exposed  to  hot  dry  air,  are  probably 
protective  against  both  light  and  heat  rays,  and  the 
same  is  true  of  the  vertically  placed  leaves  of  Eu- 
calyptus and  Manzanita.  This  also  holds  good  for 
the  great  reduction  of  surface  seen  in  many  plants 
of  arid  regions.  In  some  species  of  broom  and 
asparagus,  and  the  cacti,  the  leaves  are  nearly  or 
quite  absent,  and  the  small  twigs  develop  chlorophyll 
in  their  outer  tissues  and  replace  the  leaves.  The 
amount  of  green  tissue  is  thus  greatly  reduced,  but 
the  activity  of  these  cells  is  much  greater  owing 
to  the  more  intense  light,  and  at  the  same  time  the 
transpiration  surface  is  correspondingly  reduced. 

Certain  effects  apparently  due  directly  to  other 
factors  may  be  found  ultimately  to  be  the  result  of 
light.  Thus  it  has  been  found  that  plants  having 
bright-colored  flowers,  when  forced  into  bloom  at 
high  temperatures  in  a  greenhouse,  will  develop  pale- 
colored  or  even  white  flowers.  Klebs,  and  others 


Environment  and  Adaptation        211 

who  have  investigated  this  phenomenon,  have 
pointed  out  that  the  failure  to  develop  the  normal 
color  in  the  flower  may  be  attributed,  not  directly 
to  the  chemical  effect  of  the  light,  but  to  the  ex- 
haustion of  formative  materials  due  to  rapid  growth 
incited  by  the  high  temperature.  The  photosyn- 
thetic  activity,  owing  to  the  weakness  of  the  illumi- 
nation, is  not  sufficient  to  provide  the  extra  material 
needed  for  the  development  of  the  normal  pigment 
in  the  flowers,  but  this  is  used  up  in  the  growth  of 
the  plant.  If  the  same  plants  are  grown  where  it 
is  cooler,  and  growth  therefore  less  active,  while 
the  illumination  is  equally  strong,  pigment  will  re- 
appear in  the  flowers.  The  light,  therefore,  does 
not  directly  cause  the  production  of  the  pigment, 
but,  by  promoting  photosynthesis,  allows  for  the  ac- 
cumulation of  the  substances  necessary  for  the  de- 
velopment of  the  pigment. 

FUNGI 

Fungi  Either  Parasites  or  Saprophytes. — What- 
ever may  have  been  the  origin  of  the  Fungi,  they 
differ  radically,  both  in  their  structure  and  habits, 
from  the  green  plants,  and  show  many  unmistakable 
instances  of  special  adaptations.  They  subsist 
largely  upon  solid  organic  matter,  such  as  the  liv- 
ing bodies  of  plants  and  animals,  or  dead  substances 
like  decayed  wood  and  vegetable  mold.  The  plant 
body  consists  of  fine  filaments,  or  hyphae,  which  by 


212  Plant  Life  and  Evolution 

the  development  of  active  ferments,  or  enzymes,  are 
able  to  penetrate  the  most  resistant  organic  sub- 
stances, like  wood,  or  the  chitinous  armor  of  insects. 
This  ability  to  destroy  organic  bodies  makes  the 
fungi  of  some  importance  in  the  decomposition  of 
organic  matter,  although  their  role  in  this  respect 
is  much  less  important  than  that  of  the  bacteria. 

Many  of  the  parasitic  fungi,  like  the  black  knot 
of  plums,  or  the  onion  mildew,  cause  abnormal 
growths,  sometimes  resembling  the  galls  formed  by 
insects.  Whether  these  hypertrophied  growths  are 
due  to  mechanical  irritation,  or  to  the  effect  of  en- 
zymes secreted  by  the  fungus,  or  to  some  other 
chemical  stimulus,  is  not  certain.  But  the  abnormal 
growth  is  presumably  advantageous  to  the  parasite, 
as  the  food  supply  must  in  this  way  be  notably 
increased. 

Symbiosis. — While  most  parasitic  fungi  are  very 
destructive  to  their  hosts,  sometimes  killing  them 
outright,  there  is  a  modified  form  of  parasitism 
which  is  of  common  occurrence.  This  is  known  as 
Symbiosis,  and  is  a  phenomenon  of  much  wider 
occurrence  than  was  formerly  supposed.  The  nitro- 
gen-fixing bacteria  have  been  referred  to  in  a  former 
chapter,  and  it  now  seems  certain  that  a  considerable 
number  of  fungi  can  also  utilize  free  nitrogen,  and 
are  of  material  assistance  in  supplying  nitrogen  to 
certain  plants  with  which  they  live  symbiotically. 
They  are  often  associated  with  the  roots  of  many 
flowering  plants,  especially  certain  trees  like  the 


Environment  and  Adaptation        213 

beech,  and  many  species  growing  in  humus,  such  as 
some  of  the  Heath  family,  e.g.,  species  of  rhododen- 
dron and  huckleberries.  In  such  saprophytes  as  the 
Indian-pipe  (Monotropa),  and  the  coral-root  or- 
chids, which  are  without  chlorophyll,  the  fungi  fur- 
nish not  only  nitrogen,  but  also  carbon  in  some  form. 
It  seems  likely  that  the  carbonaceous  matter  from 
the  humus  is  first  elaborated  by  the  fungus,  which  is 
then  itself  destroyed  within  the  tissues  of  the  host. 
There  seems  to  be  a  sort  of  mutual  parasitism.  The 
fungus  at  first  feeds  upon  the  host,  which  after- 
wards retaliates,  and  destroys  the  fungus  within  its 
tissues. 

The  most  familiar  case  of  symbiosis  is  that  of  the 
lichens,  where  a  fungus  and  an  alga  are  intimately 
associated.  While  in  this  association  the  fungus  un- 
doubtedly behaves  as  a  true  parasite  toward  the  alga, 
which  under  favorable  conditions  can  grow  quite 
independently,  there  seems  no  reason  to  doubt  that 
the  alga  itself  derives  some  benefit  from  its  associa- 
tion with  the  fungus.  Within  the  sheltering  tangle 
of  fungus  filaments  it  is  supplied  with  water,  and  it 
is  quite  probable  that  a  certain  amount  of  nourish- 
ment, presumably  of  a  nitrogenous  nature,  is  also 
supplied  to  it. 

Synchytrium  papillatum. — That  many  parasitic 
fungi  are  of  comparatively  recent  origin  is  certain, 
as  some  species  may  be  associated  with  a  specific 
host,  which  is  often  a  highly  organized  and  pre- 
sumably a  recent  type  of  flowering  plant,  and  the 


214  Plant  Life  and  Evolution 

specific  characters  of  the  parasite  must  have  been 
developed  as  late,  at  least,  as  those  of  the  host.  A 
curious  case  is  that  of  the  parasitic  fungus,  Syn- 
chytrium  papillatum,  which  at  present  is  known  only 
from  California,  where  it  sometimes  grows  abun- 
dantly upon  a  weed,  Erodium,  which  is  supposed 
to  have  been  introduced  from  Europe,  not  more 
than  two  hundred  or  three  hundred  years  ago. 
The  fungus  is  not  known  to  occur  in  Europe,  nor 
has  it  been  found  upon  any  native  Calif ornian  plant. 
It  would  be  interesting  to  know,  whether  we  have 
a  new  species  arising  from  some  native  fungus  which 
has  adapted  itself  to  a  new  host,  and  thus  developed 
new  specific  characters,  or  whether  it  is  merely  a 
case  of  an  imported  fungus  which  has  developed 
more  luxuriantly  in  its  adopted  home. 

How  far  the  more  or  less  complete  suppression 
of  the  sexual  reproduction,  which  has  been  observed 
in  so  many  of  the  fungi,  is  due  to  degeneration, 
consequent  upon  their  habits,  is  not  possible  to  de- 
termine. 

STORAGE  ORGANS 

In  the  economy  of  the  plant  it  often  becomes 
necessary  to  provide  storage  organs  upon  which 
the  plant  can  draw  at  certain  times.  This 
is  seen  in  its  simplest  form  in  the  spores  of 
the  lower  plants,  which  are  packed  with  various 
nutritive  substances  like  starch  and  oil,  which  fur- 


Environment  and  Adaptation        215 

nish  the  materials  for  the  first  stages  of  germina- 
tion. In  the  higher  plants  the  seeds  contain  similar 
stores  of  food,  either  in  the  endosperm  or  less  com- 
monly in  the  outer  seed-tissues,  or  in  the  tissues  of 
the  embryo  itself.  Other  types  of  storage  organs 
are  bulbs,  tubers,  root-stocks,  etc. 


PARASITES  AND  SAPROPHYTES 

The  great  group  of  fungi  are  all  either  para- 
sites or  saprophytes,  and  among  the  flowering 
plants  there  are  also  found  many  species  which 
have  become  more  or  less  completely  parasitic 
or  saprophytic,  but  the  latter  are  probably 
all  derived  from  forms  originally  possessing  chloro- 
phyll. Parasites  and  saprophytes  are  not  common 
among  the  archegoniates,  but  there  are  a  num- 
ber of  examples  known.  Where  plants  are  com- 
pletely parasitic  or  saprophytic  there  is  a  com- 
plete loss  of  chlorophyll,  and  with  it  a  more  or  less 
extensive  degeneration  of  the  leaves.  The  humus 
saprophytes  may  also  have  their  roots  replaced  by 
root-like  stems.  These  differences  are  evidently  cor- 
related with  the  marked  changes  in  the  method  of 
nutrition. 

Parasitic  Flowering  Plants. — The  normal  green 
plant  derives  most  of  its  food  from  the  inorganic 
substances  CO2  and  water,  together  with  certain 
nitrogenous  and  other  elements  absorbed  from  the 
soil.  There  are,  however,  many  plants  which  are 


216  Plant  Life  and  Evolution 

dependent  to  a  greater  or  less  degree  upon  other 
organisms  for  their  substance.  It  is  among  the 
higher  flowering  plants  that  the  most  remarkable 
parasites  and  saprophytes  occur.  The  most  extreme 
parasites  are  found  in  certain  tropical  and  sub- 
tropical species,  of  which  the  extraordinary  Raf- 
flesia  of  Sumatra  may  be  taken  as  a  type.  This 
parasite  passes  the  whole  of  its  vegetative  existence 
within  the  tissues  of  its  host,  a  species  of  wild  vine, 
and  the  vegetative  structures  of  the  parasite  are  so 
reduced,  that  they  more  nearly  resemble  the  myce- 
lium of  a  fungus  than  the  body  of  a  normal  flower- 
ing plant,  and  the  parasite  feeds  upon  its  host  some- 
what in  the  same  way  that  the  fungus  does,  the 
tissues  being  in  direct  communication  with  the  con- 
ducting tissues  of  the  host.  At  maturity  enormous 
flower  buds  are  formed,  which  burst  through  the 
outer  tissues  of  the  host,  and  the  gigantic  flower 
expands  and  develops  its  seeds  exposed  to  the  air. 
The  extraordinary  degradation  of  these  endo-para- 
sitic  flowering  plants  makes  it  very  difficult  to  ascer- 
tain their  relationships.  Complete  parasitism,  but 
less  extreme,  is  met  with  in  a  good  many  more  or 
less  familiar  plants.  The  dodder  is  one  of  the 
best-known  forms,  and  is  a  genuine  parasite  upon 
a  variety  of  other  plants,  twining  its  leafless  stems 
about  them,  and  sending  suckers  into  the  host 
from  which  it  derives  all  its  nourishment.  The 
leaves  are  reduced  to  scales  and  only  a  slight  trace 
of  chlorophyll  can  be  detected.  Many  of  these  para- 


Environment  and  Adaptation        217 

sites  grow  upon  the  roots  of  other  plants,  e.g., 
beech-drops  (Epiphegus),  canker-root  (Aphyllon), 
and  others.  Certain  parasites  may  not  be  entirely 
dependent  upon  their  host  for  carbon,  being  able 
to  assimilate  CO2.  Of  these  parasites  the  various 
species  of  mistletoe  are  the  best-known  examples, 
and  a  number  of  the  Figwort  family  are  also  known 
to  be  root  parasites,  although  these  have  well-devel- 
oped roots.  Of  these  green  root-parasites  Gerardia 
and  Castilleia  may  be  mentioned. 

Saprophytes. — There  are  many  saprophytic  flow- 
ering plants,  these  being  especially  numerous  in  the 
Heath  family,  and  among  the  orchids.  Sapro- 
phytism  may  be  present  in  plants  having  green 
leaves,  like  species  of  rhododendron  and  many 
others ;  while  in  others  all  chlorophyll  has  been  lost, 
and  the  leaves  and  sometimes  the  roots  are 
rudimentary.  The  coral-root  orchids,  the  Indian- 
pipe,  and  the  curious  snow-plant  (Sarcodes),  of 
the  Sierra  Nevada,  represent  this  extreme  case  of 
saprophytism.  In  all  of  these,  so  far  as  they  have 
been  investigated,  there  is  always  associated  a 
fungus,  by  means  of  which  they  seem  to  be  able  to 
utilize  the  necessary  carbon  compounds  from  the 
humus.  This  peculiar  form  of  parasitism,  symbi- 
osis, has  already  been  referred  to.  The  exact  nature 
of  this  association  is  not  always  clear,  but  its  con- 
stant occurrence  implies  that  the  association  is  mu- 
tually beneficial.  In  the  lichens  the  association  is 
so  intimate  that  the  resulting  structure  has  assumed 


218  Plant  Life  and  Evolution 

the  form  and  character  of  a  distinct  organism. 
There  is  no  doubt  that  the  fungus  element  is  para- 
sitic upon  the  alga,  upon  which  it  is  dependent  for 
its  existence,  but  the  alga  seems  to  suffer  little  from 
its  imprisonment  in  the  tissues  of  the  fungus,  and  as 
the  latter  takes  up  water  very  quickly  and  retains  it 
tenaciously,  the  alga  is  undoubtedly  enabled  to  grow, 
thanks  to  the  shelter  of  the  fungus,  where  otherwise 
it  could  not  exist.  Moreover,  the  recent  evidence 
that  some  of  the  higher  fungi,  as  well  as  bacteria, 
can  assimilate  nitrogen,  makes  it  quite  probable  that 
the  fungus  gives  to  the  alga  certain  nitrogenous 
compounds  in  return  for  the  carbonaceous  food 
taken  from  it.  It  is  likely  that  in  the  case  of  the 
association  of  the  fungus  with  a  saprophytic  flow- 
ering plant,  nitrogen  is  also  furnished  to  the  host  as 
well  as  carbon. 

The  symbiotic  association  of  two  green  plants  is 
much  less  common,  but  a  good  many  cases  are 
known.  Usually  one  of  the  symbionts  is  a  blue- 
green  alga,  and  it  is  possible  that  here  also  there 
may  be  a  case  of  nitrogen  assimilation  which  may 
be  useful  to  the  other  party  of  the  association.  A 
number  of  liverworts,  e.g.,  Blasia,  Anthoceros, 
have  always  associated  with  them  a  species  of 
Nostoc,  and  the  little  water-fern,  Azolla,  always 
harbors  in  its  leaves  colonies  of  a  blue-green  alga 
(Anabaena).  No  actual  parasitism  has  been  shown 
in  any  of  these  cases,  and  just  what  the  relation  of 
the  two  symbionts  is  we  really  do  not  know. 


Environment  and  Adaptation        219 

Carnivorous  Plants. — A  most  extraordinary  form 
of  adaptation  is  that  of  the  so-called  carnivorous 
plants,  of  which  a  number  of  remarkable  types  are 
common  in  the  United  States.  These  are  either 
aquatics  or  bog  plants,  and  it  is  supposed  that  their 
peculiar  habits  are  due  to  a  deficiency  of  nitrogen 
in  their  environment.  In  the  sundew  (Drosera), 
butterwort  (Pinguicula),  and  Venus's  flytrap  (Di- 
onaea)  the  leaves  are  modified  into  traps,  which 
capture  small  insects  alighting  upon  them,  and  after 
the  insect  is  secured,  there  is  an  actual  digestion  by 
the  aid  of  digestive  ferments  not  unlike  those  found 
in  the  digestive  organs  of  animals.  A  similar  fer- 
ment has  been  demonstrated  in  the  pitchers  of  the 
Asiatic  pitcher-plant  (Nepenthes).  In  the  Amer- 
ican pitcher-plants  (Sarracenia  and  Darlingtonia), 
and  in  the  bladder- weed  (Utricularia),  the  leaves 
form  traps  into  which  the  insects  are  lured,  but 
there  is  very  little  or  no  digestive  effect  upon  the 
bodies  of  the  victims,  which  are  drowned  and  the 
products  of  their  decomposition  are  absorbed  by  the 
leaf.  These  carnivorous,  or  insectivorous,  plants  are 
among  the  most  extraordinary  examples  of  special 
adaptation  that  are  met  with  in  the  whole  vegetable 
kingdom. 

PLANTS  AND  ANIMALS 

As  plants  are  essential  to  the  existence  of  all  ani- 
mal life,  it  is  not  remarkable  that  the  structures  of 


22O  Plant  Life  and  Evolution 

animals  should  often  show  evident  adaptations  to 
plant  structures.  The  mouth  parts  and  the  digestive 
organs  of  herbivorous  animals,  the  beak  and  tongue 
of  humming-birds,  the  mouth  part  of  many  insects, 
are  a  few  of  the  most  common  examples.  But  it  is 
equally  clear  that  certain  modifications  of  plants 
have  also  been  induced  by  their  relation  to  animals. 
The  most  remarkable  of  these  adaptations  are  asso- 
ciated with  the  employment  of  animals  as  agents  in 
pollination  and  seed  distribution. 

Of  course  it  is  impossible  to  say  just  how  far  the 
relation  of  animals  to  plants  has  acted  directly  upon 
the  structures  of  the  latter,  but  it  is  very  evident 
that  whatever  may  have  been  the  inducement  of  cer- 
tain structures,  the  preservation  and  perfecting 
of  these  characters  have  been  very  potent  factors  in 
aiding  the  plants  in  the  struggle  for  existence.  Thus 
the  grasses  have  been  extraordinarily  successful  in 
holding  their  own  in  competition  with  other  plants 
in  nearly  all  parts  of  the  world,  and  at  the  same 
time  they  form  perhaps  the  most  important  of  all 
food  plants  for  the  higher  animals.  In  common 
with  many  other  monocotyledons,  the  leaves  fre- 
quently possess  an  unlimited  power  of  basal  growth, 
and  may  be  cropped  repeatedly  without  injury; 
moreover,  many  grasses  are  characterized  by  an  ex- 
traordinary power  of  rapidly  spreading  by  means 
of  underground  stems  or  runners.  It  would  per- 
haps be  rash  to  assert  that  these  habits  have  arisen 
in  response  to  a  need  for  protection  against  the  rav- 


Environment  and  Adaptation        221 

ages  of  grazing  animals,  but  there  is  no  question 
that  these  peculiarities  have  enabled  the  plants  to 
survive  and  flourish,  in  spite  of  constant  cropping. 

Protection  Against  Animals. — Somewhat  differ- 
ent are  the  special  protective  devices  found  in  many 
plants  which  enable  them  to  repel  the  attacks  of  ani- 
mals. These  are  especially  marked  in  plants  of  arid 
regions  where  the  life  conditions  are  precarious. 
The  development  of  defensive  armor,  like  the  terri- 
ble spines  of  the  cacti,  and  the  dagger  leaves  of  the 
century  plant  and  yuccas,  as  well  as  the  rank  secre- 
tions of  the  sage-brush  and  creosote-bush,  are  very 
efficient  weapons  against  the  attacks  of  hungry  ani- 
mals ;  and  although  they  may  be  only  physiological 
responses  to  the  arid  environment,  they  are  never- 
theless exceedingly  useful  to  the  plant  as  protective 
measures,  and  must  have  been  of  immense  impor- 
tance in  preserving  plants  in  the  very  unfavorable 
conditions  surrounding  them. 

Myrmecophily. — The  insectivorous  habit  of  cer- 
tain plants  has  already  been  referred  to,  but  another 
extraordinary  association  with  insects  may  be  briefly 
cited,  as  it  is  one  of  the  most  remarkably  reciprocal 
adaptations  known  to  the  naturalist.  This  is  the 
habit  discovered  in  certain  ants  of  associating  them- 
selves with  plants  in  a  sort  of  symbiotic  relation, 
which  has  been  termed  "  myrmecophily."  In  a 
number  of  trees,  notably  the  genus  Cecropia  in 
tropical  America,  and  certain  species  of  Acacia, 
the  trees  harbor  colonies  of  ants  which  inhabit 


222  Plant  Life  and  Evolution 

their  hollow  stems,  or  in  the  case  of  the  Acacia, 
much  enlarged  hollow  thorns.  These  trees  are  sub- 
jected to  the  attacks  of  leaf-cutting  ants,  which  are 
repelled  by  the  ants  living  in  the  trees,  whose  foliage 
is  thus  saved  from  destruction.  Sometimes  the  trees 
furnish  not  only  lodging  but  also  board,  as  there  are 
developed  certain  peculiar  secretions  which  serve  as 
food  for  their  insect  tenants. 

A  remarkable  form  of  myrmecophily  has  been 
recently  studied  by  Wheeler  in  a  number  of  ants 
from  Texas  and  other  warmer  parts  of  America. 
These  ants  carry  into  their  nests  masses  of  leaf- 
fragments  which  they  pack  together  so  as  to  form  a 
sort  of  miniature  hotbed.  Upon  this  mass  of  fer- 
menting vegetation  there  soon  appears  a  peculiar 
fungus,  which  grows  luxuriantly  and  produces  food 
bodies  upon  which  the  ants  feed.* 

REPRODUCTION 

The  Necessity  for  Reproduction. — The  necessity 
of  some  form  of  propagation  for  the  perpetuation  of 
the  species  is  evident,  and  many  types  of  reproduc- 
tion have  developed  in  response  to  this  need.  The 
simplest  of  all  is  the  ordinary  cell  fission,  the  only 

*  Professor  Wheeler,  in  his  recent  book  on  the  structure  and 
habits  of  ants,  has  expressed  some  doubt  as  to  the  entire  ac- 
curacy of  some  of  the  observations  made  upon  the  habits  of 
the  tree-dwelling  ants.  He  believes  that  the  adaptations  be- 
tween the  ants  and  the  host  tree  are  not  so  complete  as  have 
been  assumed. 


Environment  and  Adaptation        223 

type  of  reproduction  in  many  of  the  lowest  organ- 
isms. Usually,  however,  there  are  developed  more 
or  less  specialized  cells,  whose  sole  function  is  repro- 
duction. In  many  algae  these  reproductive  cells  es- 
cape from  the  parent  cell  and  become  free-swimming 
zoospores,  which  settle  down  and  grow  into  new 
plants  directly.  The  advantage  of  the  motile  condi- 
tion for  the  distribution  of  the  species  is  evident 
enough. 

Sudden  changes  in  the  environment  may  act 
as  powerful  stimuli  in  inducing  the  formation 
of  reproductive  organs.  It  has  been  shown  that 
algae  in  running  water  grow  vigorously,  but  seldom 
or  never  develop  their  reproductive  cells;  but  when 
transferred  to  still  water  they  will  often  develop  zo- 
ospores in  very  great  numbers.  Some  algae  which 
live  in  the  air  will  quickly  form  zoospores  on  trans- 
ferring them  to  water.  Light,  temperature,  changes 
in  the  food — all  of  these  act  as  stimuli  in  controlling 
the  reproductive  processes.  Thus  in  the  water-net 
(Hydrodictyon),  plants  grown  in  a  solution  of  cane 
sugar  will  produce  an  enormous  number  of  gametes. 

While  in  many  of  the  lower  plants  reproduction 
is  purely  non-sexual,  most  algae  produce  some 
form  of  gametes,  or  sexual  cells.  These  are  evi- 
dently modifications  of  originally  non-sexual  cells, 
or  zoospores,  and  it  is  sometimes  impossible 
to  certainly  distinguish  between  the  two.  While 
usually  there  is  a  fusion  of  the  gametes,  they 
may  develop  without  union  under  certain  conditions, 


224  Plant  Life  and  Evolution 

and  this  parthenogenesis  may  be  the  result  of 
changed  conditions;  for  example,  increase  of  tem- 
perature, or  cultivation  in  a  special  nutritive  medium. 

Resting  Spores. — Very  commonly  the  zygote  re- 
sulting from  the  union  of  the  gametes  is  a  resting 
cell,  or  spore,  which  is  adapted  to  resist  the  desicca- 
tion to  which  fresh-water  algae  are  so  frequently 
exposed.  It  may  be  assumed  that  the  various  forms 
of  thick-walled  resting  cells,  developed  by  fresh- 
water algae,  are  adaptations  to  drying  up,  as  these 
are  very  seldom  met  with  in  the  marine  algae, 
where  drying  up,  of  course,  never  occurs.  The  con- 
nection of  these  resting  spores  with  the  origin  of 
the  lower  land  plants  has  already  been  pointed  out. 

Spore  Distribution  by  Insects.' — It  is  in  connec- 
tion with  reproduction  that  plants  have  been  most 
profoundly  influenced  in  their  structures  through 
association  with  animals.  Among  the  lower  plants 
this  is  far  less  marked  than  is  the  case  among  the 
flowering  plants,  but  there  are  a  number  of  cases 
of  apparent  adaptation  to  animal  agencies  in  spore 
distribution.  Of  course  the  accidental  distribution 
of  spores  and  other  minute  germs,  which  adhere 
to  the  bodies  of  animals,  must  constantly  occur,  but 
there  are  several  cases  where  this  seems  to  be  es- 
pecially provided  for.  Thus  in  the  fungus  which 
causes  the  disease  known  as  Ergot  upon  rye,  at  a 
certain  stage  in  the  development  of  the  fungus  there 
is  produced  a  sweet  substance  which  attracts  in- 
sects, to  which  the  spores  adhere  and  are  presuma- 


Environment  and  Adaptation        225 

bly  thus  disseminated.  The  slimy  spore  masses  of 
the  Phalloidese,  a  family  of  large  fungi,  have  an 
excessively  offensive  odor  which  is  said  to  attract 
carrion-loving  insects,  which  are  the  disseminators 
of  the  spores  which  adhere  to  their  bodies.  It  is 
among  the  angiosperms,  however,  that  the  most 
perfect  instances  of  these  adaptations  are  found. 

Cross-pollination  by  Insects. — While  there  has 
lately  been  a  tendency  to  minimize  the  importance 
of  insect  aid  in  the  pollination  of  flowers,  and  to 
explain  otherwise  their  remarkable  color  devices  and 
structures,  there  can  be  no  question  that  the  extraor- 
dinary development  and  diversity  of  the  angio- 
sperms is,  in  a  very  large  measure,  the  result  of  their 
adaptations  to  cross-pollination  through  insect 
agency.  Cross-pollination  is  known  to  be  distinctly 
advantageous  in  many  cases.  The  seeds  of  cross- 
pollinated  flowers  have  been  shown  by  Darwin  and 
other  investigators  to  be  more  numerous  and  better 
developed,  and  the  resulting  seedlings  distinctly 
larger  and  more  vigorous,  than  those  derived  from 
seeds  from  self-pollinated  flowers.  Moreover,  a 
good  many  flowers,  e.g.,  many  orchids,  have  been 
found  to  be  quite  sterile  with  their  own  pollen, 
which  may  even  act  injuriously  upon  the  pistil.  It 
is  also  a  legitimate  assumption  that  the  increased 
variability  due  to  cross-pollination  is  an  advantage, 
as  tending  to  cause  new  characters  to  appear  which 
may  be  taken  advantage  of  by  natural  selection. 

The  lower  types  of  flowers,   such  as  those  of 


226  Plant  Life  and  Evolution 

nearly  all  gymnosperms  and  the  apetalous  angio- 
sperms  like  the  oaks  and  most  grasses,  are  gen- 
erally dependent  upon  the  wind  for  distributing  their 
pollen,  which  is  light  and  produced  in  very  great 
quantities,  and  is  readily  borne  long  distances 
through  the  air.  Cross-pollination  is  often  the  rule, 
however,  even  here,  as  the  flowers  are  frequently 
"  diclinous,"  that  is,  have  their  stamens  and  carpels 
in  different  flowers.  Wind  pollination  involves  a 
great  waste  of  pollen,  as  probably  not  one  pollen- 
spore  in  a  million  is  efficacious.  It  is  clear  that  a 
material  saving  in  the  amount  of  pollen  and  its  in- 
creased efficiency  ought  to  be  of  advantage  to  the 
plant. 

Pollen  cells  are  rich  in  nutritive  matter,  and  hence 
are  sought  for  as  food  by  many  insects.  It  may 
be  supposed  that  the  first  cases  of  insect  pollination 
were  purely  accidental  and  brought  about  by  the 
search  for  pollen  as  food.  If  for  any  reason  any 
flowers  should  be  more  conspicuous  than  others,  it 
is  quite  conceivable  that  they  would  more  readily 
attract  the  attention  of  visiting  insects,  and  it  is 
quite  conceivable  also  that  through  some  increased 
size  of  the  enveloping  leaves,  or  brighter  color  of 
the  stamens,  the  line  of  evolution  started  which 
culminated  in  the  gorgeously  colored  and  highly 
specialized  flowers  of  many  of  the  orchids  and  Com- 
positse. 

In  the  lower  types  of  flowers  the  enveloping 
leaves  are  inconspicuous  scales,  serving  merely 


Environment  and  Adaptation        227 

for  protective  purposes;  but  we  soon  meet  with 
flowers  in  which  these  are  replaced  by  more  or 
less  conspicuous  floral  leaves.  It  is  safe  to  say 
that  no  showy  flower  is  entirely  destitute  of 
insect  visitors,  although  it  may  not  be  abso- 
lutely dependent  upon  them  for  its  pollination,  and 
cross-pollination  must  occur  in  a  great  many 
cases.  If  cross-pollination  is  prevented,  however, 
many  flowers  are  capable  of  pollinating  them- 
selves. Such  flowers  as  the  buttercup  or  anemone 
and  the  inflorescences  of  many  Compositae,  like  the 
dandelion,  are  of  this  character.  In  the  latter  case, 
however,  cross-pollination  of  a  sort  really  does  oc- 
cur, as  we  have  to  do,  not  with  a  single  flower,  but 
with  a  group  of  flowers  in  which  each  individual 
flower  is  likely  to  be  pollinated  from  another  one. 

Specialization  of  the  Flower. — In  the  simpler 
hermaphrodite  floral  types,  such  as  the  water-lily 
or  magnolia,  there  is  a  multiplication  of  parts  and 
an  indefiniteness  in  their  number  that  is  in  strong 
contrast  to  the  very  definite  structures  of  such  a 
flower  as  a  foxglove  or  orchid.  This  definiteness 
of  structure  involves  a  reduction  in  the  number  of 
certain  parts  (see  chapter  on  Angiosperms  for  de- 
tails), and  later  a  cohesion  of  the  floral  organs. 
This  begins  with  the  carpels,  which  in  a  majority  of 
the  higher  plants  are  fewer  in  number  than  the 
other  organs,  and  are  more  or  less  completely  united 
into  a  compound  pistil.  Next  follows  the  reduction 
in  the  number  of  stamens,  which  reaches  its  maxi- 


228  Plant  Life  and  Evolution 

mum  in  certain  highly  specialized  monocotyledons 
like  the  orchids  and  canna,  where  usually  only  a 
single  functional  stamen  is  present.  Where  the 
number  of  stamens  is  reduced,  this  is  almost  always 
associated  with  change  from  the  original  radial 
symmetry  of  the  flower  to  a  marked  bilateral  sym- 
metry. This  is  seen  in  the  cases  cited  and  also  in 
such  dicotyledons  as  the  mints  and  bignonias,  in 
which  the  floral  leaves  are  united  into  a  tubular  or 
trumpet-shaped  corolla. 

Color  and  Scent  in  Flowers. — With  these  modifi- 
cations in  structure  there  are  associated  a  great  va- 
riety of  vivid  colors,  so  that  these  specialized  flow- 
ers include  most  of  the  more  showy  species  under 
cultivation.  Another  common  phenomenon  is  the 
development  of  the  characteristic  scents  in  flowers, 
these  being,  in  the  opinion  of  many  modern  students 
of  cross-pollination,  the  most  potent  means  of  at- 
tracting insect  visitors.  Color  and  scent  may  both 
be  associated  with  the  secretion  of  nectar,  the  prin- 
cipal object  of  the  visits  of  butterflies  and  many 
other  insects. 

How  far  insects  are  able  to  discriminate  the 
colors  of  flowers,  and  especially  the  elaborate 
color-patterns  of  the  so-called  "  honey  guides," 
etc.,  is  much  in  need  of  thorough  investigation. 
While  the  earlier  investigators  attributed  to  insects 
a  capacity  for  color  discrimination  not  entirely 
warranted  by  the  facts,  there  is  no  question 
that  the  conclusions  of  some  of  the  recent  oppo- 


Environment  and  Adaptation        229 

nents  of  these  views  go  too  far  in  the  other  direc- 
tion, and  are  quite  as  much  in  need  of  further  con- 
firmation. It  may  be  safely  asserted  that  any  showy 
flower  is  normally  pollinated  by  insects,  and  there  is, 
moreover,  no  question  that  color  is  in  some  cases 
directly  an  adaptation.  The  pale  color  of  most 
night-blooming  flowers  is  an  obvious  adaptation, 
making  the  flowers  more  conspicuous  in  the  dark 
or  twilight,  and  it  is  equally  evident  that  the  strong 
scent  of  many  of  these  nocturnal  flowers  is  also  a 
powerful  lure  to  the  insects  visiting  them.  Some 
of  the  nocturnal  flowers,  which  are  quite  scentless 
in  the  daytime,  exhale  a  very  strong  perfume  in 
the  evening.  This  is  especially  true  in  some  of  the 
Nightshade  family.  The  long-tubed  flower  of  the 
white  petunia  and  the  white-flowered  tobacco,  often 
grown  as  an  ornamental  plant,  flood  the  garden  with 
their  strong  perfume  soon  after  sunset,  and  the 
odor  wafted  from  the  big  trumpets  of  the  tree 
Datura  is  almost  overpowering.  These  tubular 
nocturnal  flowers  are  especialy  frequented  by  the 
great  humming-bird  moths,  whose  enormously  long 
tongues  are  especially  fitted  for  probing  their  nec- 
taries. 

While  such  showy  flowers  as  the  magnolias 
and  buttercups,  which  are  normally  insect  polli- 
nated, can,  if  necessary,  pollinate  themselves,  this 
is  not  the  case  in  a  great  many  flowers,  which  are 
so  constructed  that  cross-pollination  is  absolutely 
necessary.  Only  a  brief  reference  can  be  made  to 


230  Plant  Life  and  Evolution 

a  few  of  the  more  striking  cases.  For  a  fuller 
account  of  these  special  contrivances  to  insure  cross- 
pollination  the  reader  must  be  referred  to  the  works 
of  Darwin,  Kerner,  Miiller,  and  other  students  of 
these  interesting  problems. 

Prevention  of  Self-fertilization. — One  of  the  com- 
mon methods  by  which  cross-pollination  is  secured, 
is  the  maturing  at  different  times  of  the  stamens  and 
carpels.  A  common  example  of  this  is  seen  in  the 
scarlet  geranium,  where  all  the  pollen  is  shed  before 
the  pistil  is  ready  to  receive  it,  so  that  the  flower 
must  be  pollinated  from  a  younger  one,  and  this 
must  be  done  through  the  aid  of  insects.  In  the 
nasturtium  much  the  same,  conditions  exist,  but  the 
pistil,  when  it  is  ready  to  receive  the  pollen,  takes 
a  position  exactly  the  same  as  that  occupied  by 
the  stamens  at  the  time  that  the  pollen  is  shed,  so 
that  the  bee  or  humming-bird,  coming  from  the 
younger  flower,  and  bearing  with  it  the  pollen, 
touches  the  same  part  of  the  body  to  the  pistil  in 
the  older  flower,  and  thus  deposits  upon  it  the  pollen 
which  it  has  brought  from  the  younger  one  (Fig. 
21,  C,  D). 

Heterostylism. — In  a  number  of  plants,  including 
species  of  primrose  and  some  of  our  native  plants, 
e.g.,  the  trailing  arbutus  and  partridge-berry,  what 
is  known  as  heterostylism  occurs,  i.e.,  there  are  pis- 
tils of  two  lengths  in  different  flowers,  and  the 
stamens  are  of  reciprocal  lengths.  Thus  long- 
styled  flowers  have  short  stamens,  and  vice  versa. 


Environment  and  Adaptation        231 

This  usually  ensures  the  pollination  of  the  pistil  of 
a  flower  with  pollen  from  stamens  of  corresponding 
length,  taken  from  another  flower. 


FIG.  21 

A — Recently  opened  flower  of  Pelargonium ;  the  stamens 
are  ready  to  discharge  the  pollen,  but  the  stigma,  st,  is  still 
immature. 

B — An  older  flower  of  Pelargonium,  with  the  petals  removed ; 
the  anthers  have  fallen,  and  the  stigma  is  open  and  ready  for 
pollination. 

C — Section  of  a  young  flower  of  Nasturtium;  three  of  the 
seven  stamens  have  discharged  their  pollen;  the  others  are 
nearly  ready  to  do  so,  but  the  pistil,  st,  is  still  immature. 

D — Older  flower ;  all  the  stamens  have  discharged  the  pollen 
and  bent  down ;  the  ripe  pistil  now  occupies  a  position  in  front 
of  the  open  spur,  where  it  will  receive  pollen  brought  from  a 
younger  flower. 

Cross-pollination  in  Orchids. — The  orchids  prob- 
ably show  the  most  extraordinary  adaptations  for 


232 


Plant  Life  and  Evolution 


cross-pollination,  their  flowers  being  often  abso- 
lutely sterile  unless  the  proper  insect  visitors  are 
available.  One  of  the  simplest  cases  is  seen  in 
the  genus  Orchis  (Fig.  22).  In  Orchis,  and  the 
same  is  true  of  very  many  other  orchids,  the 


FIG.  22 

A — Flower  of  Orchis  spectabilis;  L,  the  lip ;  gy,  the  column 
or  gynostemium. 

B — A  flower  with  the  upper  segments  bent  back  so  as  to  ex- 
pose the  column ;  an,  the  anther ;  d,  adhesive  disc  of  the  pol- 
linium;  st,  one  of  the  stigmatic  surfaces;  sp,  the  spur;  o, 
ovary. 

C — The  two  pollinia,  adhering  to  a  slender  straw  thrust  into 
the  flower. 

D — The  same  pair  of  pollinia,  a  few  minutes  later,  showing 
the  change  of  position;  if  the  straw  is  now  thrust  into  the 
flower,  the  pollinia  will  come  in  contact  with  the  stigmatic 
surfaces. 

pollen  is  aggregated  in  small  masses,  or  pollinia. 
These  pollinia  are  held  in  little  pockets,  or  recepta- 


Environment  and  Adaptation        233 

cles,  from  which  they  must  be  forcibly  removed  by 
the  agency  of  the  insect  which  visits  the  flower  for 
the  nectar.  As  the  insect  enters  the  flower,  the 
cover  of  this  receptacle  is  ruptured  and  the  packet 
of  pollen  is  withdrawn  and  adheres  firmly  to  the 
head  or  tongue  of  the  insect.  After  the  pollen  mass 
is  withdrawn,  it  often  shifts  its  position  so  that  it 
will  come  into  contact  with  the  stigmatic  surface 
of  the  next  flower  visited.  In  this  case,  as  in  most 
flowers  with  deep  nectaries  in  the  form  of  a  long 
spur,  the  honey  can  only  be  extracted  by  insects 
with  long  tongues,  like  butterflies  and  bees,  and  the 
extraordinary  mouth  parts  of  these  insects  are  be- 
yond any  question  adaptations  for  feeding  upon  the 
nectar  of  flowers  having  these  deep  nectaries. 

Pollination  of  Yucca. — One  more  example  must 
suffice,  as  perhaps  the  most  peculiar  adaptation  to 
cross-pollination  that  has  yet  been  studied.  In  the 
warmer  parts  of  the  United  States  there  are  several 
species  of  the  genus  Yucca,  comprising  a  number 
of  showy  lily-like  plants,  of  which  some  are  not  un- 
common in  gardens.  In  nearly  all  of  the  species 
that  have  been  studied  there  has  been  found  a  most 
extraordinary  case  of  special  adaptation,  these  plants 
usually  being  dependent  for  pollination  upon  a  sin- 
gle species  of  moth,  of  the  genus  Pronuba.  The  first 
species  which  was  described,  P.  Yuccasella,  pol- 
linates the  common  species  of  Yucca,  Y.  tilamentosa, 
in  the  Southeastern  United  States.  The  larvae  of 
these  little  moths  feed  upon  the  young  seeds  of  the 


234  Plant  Life  and  Evolution 

Yucca,  and  the  parent  moth  lays  her  eggs  in  the 
ovary  of  the  open  flower,  and  then  collects  a  mass 
of  pollen  and  forces  it  down  the  central  part  of  the 
stigma,  thus  ensuring  fertilization  of  the  ovules,  and 
the  provision  of  a  future  food  supply  for  the  larvae. 
The  latter  do  not  devour  all  of  the  seeds,  some  of 
which  are  left,  which  pay  for  the  seeds  devoured 
by  the  larvae. 

Birds  as  Agents  in  Cross-Pollination. — While 
insects  are  the  main  agents  in  cross-pollination, 
other  animals  may  be  more  or  less  important.  Oc- 
casionally snails  have  been  found  to  act  as  agents 
in  pollination,  but  next  to  insects,  certain  families 
of  birds  are  of  the  first  importance:  In  the  warmer 
parts  of  the  Old  World,  the  honey-suckers  or  sun- 
birds  of  the  large  family  Cinnyridse,  are  flower 
visitors  and  are  especially  adapted  to  extracting  the 
honey  from  flowers,  and  undoubtedly  like  insects 
they  carry  pollen  from  one  flower  to  another.  More 
important  still  are  the  humming-birds  of  the  New 
World.  They  are  distributed  practically  over  the 
whole  American  Continent,  from  Alaska  to  Pata- 
gonia. They  are  preeminently  flower  visitors,  and 
very  many  of  our  native  flowers  are  clearly  adapted 
to  their  visits.  These  "  ornithophilous  "  flowers  are 
usually  very  vividly  colored,  bright  red  seeming  to 
be  the  commonest  color.  A  host  of  tubular  scarlet 
flowers  like  the  canna,  scarlet  sage,  nasturtium, 
fuchsia,  scarlet  columbine,  trumpet  creeper,  etc., 
are  prime  favorites  of  these  little  feathered  gems. 


Environment  and  Adaptation        235 

In  his  studies  of  the  ornithophilous  flowers  of 
South  Africa,  Scott-Elliot  calls  attention  to  the  pre- 
ponderance of  bright  recj  or  orange  color  in  the 
flowers  which  are  frequented  by  the  sun-birds. 
These  belong  to  often  widely  separate  families,  both 
monocotyledons  and  dicotyledons.  He  states,  also, 
that  there  seems  to  be  a  relation  between  the  color 
of  the  birds  and  that  of  the  flowers,  the  red  color 
in  the  species  of  Cinnyris,  a  genus  of  honey-sucker, 
being  almost  exactly  identical  with  the  shade  of  red 
found  in  a  majority  of  the  ornithophilous  flowers. 
I  have  seen  myself,  in  South  Africa,  the  sun-birds 
visiting  the  scarlet  Erythrinas  and  Aloes,  and  Scott- 
Elliot  gives  a  long  list  of  other  similarly  colored 
flowers  which  are  frequented  by  these  birds.  In 
their  small  size  and  colors,  many  of  these  sun-birds 
recall  strongly  the  iridescent  American  humming- 
birds, although  they  are  not  at  all  related. 

While  we  may  hesitate  to  accept  all  the  conclu- 
sions of  the  enthusiastic  students  who  first  realized 
the  immense  importance  of  insects  in  the  pollina- 
tion of  flowers,  there  seems  to  be  no  reason  to  doubt 
that  the  course  of  evolution  of  the  two  largest 
groups  of  animals  and  plants,  insects  and  angio- 
sperms,  has  been  powerfully  influenced  by  the  mu- 
tual adaptations  that  have  arisen  in  these  two  groups 
of  organisms. 


CHAPTER  VIII 

THE  PROBLEMS  OF  PLANT  DIS- 
TRIBUTION 

THE  origin  of  the  existing  flora  of  the  earth 
is  lost  in  the  obscurity  of  an  enormously  re- 
mote past.  We  have  practically  no  knowledge  of 
the  lower  plant  types  from  the  earlier  geological 
formations,  which  is  not  surprising  when  we  re- 
member the  extreme  delicacy  of  these  very  perish- 
able organisms;  and  the  first  plant  remains  that  can 
be  identified  belong  to  species  relatively  high  up  in 
the  scale  of  development,  and  must  have  been  pre- 
ceded by  countless  forms  of  lower  rank. 

Antiquity  of  the  Principal  Types  of  Plants. — It  is 
evident  from  a  study  of  the  fossils  of  the  Paleozoic, 
that  nearly  all  of  the  living  plant  types,  except  the 
highest  forms  of  seed-plants,  were  already  in  exist- 
ence during  that  period.  Ferns  and  primitive  seed- 
plants  occur  in  the  Devonian,  and  these  reach  a 
high  degree  of  development  during  the  Carbonif- 
erous. While  few  traces  of  the  less  resistant  plant 
types,  such  as  the  seaweeds  and  mosses,  are  met 
with,  enough  of  these  have  been  found  to  show,  as 
might  have  been  expected,  that  these  plants  also 
236 


The  Problems  of  Plant  Distribution    237 

existed  during  the  early  geological  ages,  and  prob- 
ably were  not  very  different  from  their  living  de- 
scendants. 

Uniformity  of  the  Early  Floras. — A  notable  fea- 
ture of  the  primitive  floras  of  the  earth  was  their 
uniformity.  While  at  the  present  day  different 
regions  possess  very  different  floras,  during  the 
Paleozoic  era  there  seems  to  have  been  a  practically 
uniform  flora  throughout  the  greater  part  of  the 
earth.  There  is  very  little  difference  between  the 
Paleozoic  fossils  found  in  the  arctic  regions,  and 
those  which  occur  in  tropical  beds,  this  being  espe- 
cially true  of  the  Carboniferous  fossils. 

Paleozoic  Climate. — Various  explanations  of  the 
apparently  uniform  climate  that  seems  to  have  pre- 
vailed have  been  made,  one  of  the  latest,  and  one 
which  has  a  good  deal  of  plausibility,  being  that  of 
Manson,  who  believes  that  during  the  earlier  geo- 
logical epochs  the  earth  was  enveloped  in  a  dense 
layer  of  clouds  which  neutralized  the  effects  of  the 
solar  rays,  the  heat  being  mainly  the  result  of  direct 
radiation  from  the  earth  itself,  which  would  thus 
have  practically  a  uniform  climate  throughout  its 
whole  extent.  If  this  view  is  correct,  it  would  have 
to  be  assumed  that  the  cloud  envelope  was  suffi- 
ciently transparent  to  admit  enough  of  the  light 
rays  for  the  existence  of  green  plants.  But  it  must 
be  remembered  that  the  plants  of  this  period,  to 
judge  from  the  fossils,  were  mostly  forms  which 
are  able  to  grow  with  a  limited  amount  of  light,  and 


238  Plant  Life  and  Evolution 

which  do  not  require  light  for  the  development  of 
their  chlorophyll.  In  the  later  geological  time,  ac- 
cording to  this  theory,  the  layer  of  clouds  was  grad- 
ually dissipated,  and  the  zonal  climates,  as  they  now 
exist,  were  by  degrees  established.  From  a  study 
of  the  plants  of  the  Paleozoic,  especially  of  the 
Carboniferous,  which  have  left  recognizable  de- 
scendants at  the  present  time,  we  may  conclude  that 
the  climate  was  a  moist  one  but  not  necessarily  ex- 
tremely hot.  The  pteridophytes,  which  were  the 
predominant  type,  at  the  present  day  reach  their 
most  perfect  development  in  the  wet  mountain  for- 
ests of  the  tropics,  where  the  temperature  is  even, 
but  not  excessively  high. 

The  Lower  Plants  Have  Left  Few  Fossil  Re- 
mains.— The  early  history  of  the  lower  plants  can 
only  be  conjectured.  It  is  pretty  generally  conceded 
that  the  simple  green  algae  represent  more  nearly 
than  any  other  living  plants  the  ancestors  of  the 
present  land  flora.  Whatever  may  have  been  the 
origin  of  the  red  and  brown  algae,  it  is  clear  that 
they  are  relatively  modern  forms,  which  are  espe- 
cially fitted  for  marine  life.  Little  is  known  of  the 
early  geological  history  of  the  fungi,  but  traces  of 
these  are  sometimes  met  with  and  there  is  reason  to 
suppose  that  they  were  abundant  during  these  early 
times. 

Both  the  simple  green  algae  and  lower  liverworts 
show  evidences  of  their  primitive  nature,  and  prob- 
ably are  little  changed  descendants  of  their  ancient 


The  Problems  of  Plant  Distribution    239 

prototypes.  These  forms,  although  comparatively 
few  in  number  at  the  present  time,  are  of  remarka- 
bly wide  distribution,  many  of  the  genera  being 
cosmopolitan.  The  distribution  of  the  liverworts  is 
especially  interesting  in  this  connection,  and  in  most 
cases  can  be  explained  only  on  the  hypothesis  that 
they  are  survivors  of  widely  distributed  types,  which 
have  come  down  probably  from  the  Paleozoic  with 
little  change.  This  is  especially  the  case  in  such 
tropical  genera  as  Dumortiera  and  Monoclea. 

Fossil  Pteridophytes. — The  history  of  the  Pteri- 
dophytes  is  much  easier  to  trace,  as  there  are 
abundant  fossil  remains,  evidently  closely  related  to 
many  living  types,  and  indeed  some  living  genera 
can  probably  be  traced  back  to  the  Paleozoic.  The 
Paleozoic  ferns  are  for  the  most  part  of  the  so- 
called  "  eusporangiate  "  type,  and  are  allied  to  the 
living  ferns  of  the  family  Marattiaceae,  which  at 
present  are  found  mainly  in  the  tropics.  While 
many  of  the  "  ferns  "  of  the  Paleozoic  are  now 
known  to  be  seed-bearing  plants,  there  is  no  rea- 
sonable doubt  that  true  ferns,  allied  to  the  Marat- 
tiaceae,  were  abundant  in  the  Paleozoic  flora. 

Climatic  Changes  in  the  Permian. — The  last 
period  of  the  Paleozoic,  the  Permian,  was  an  era  of 
transition.  There  are  evidences  of  severe  glaciation 
in  the  Southern  Hemisphere,  and  less  marked  glacia- 
tion in  many  regions  in  the  Northern  Hemisphere, 
where  in  a  good  many  places  it  is  also  evident  that 
marked  aridity  prevailed,  in  strong  contrast  to  the 


240  Plant  Life  and  Evolution 

very  humid  climate,  which  seems  to  have  character- 
ized most  of  the  Carboniferous.  There  seem  to  have 
been  two  great  land  masses  in  existence,  a  northern 
and  a  southern  one,  and  the  floras  of  the  two  were 
different  in  many  ways.  Thus  the  Southern  Hemi- 
sphere was  characterized  by  a  type  of  fern,  Glossop- 
teris,  which  was  accompanied  by  a  number  of 
other  peculiar  southern  types  (see  Scott:  "  Studies 
in  Fossil  Botany").  It  was  probably  during  this 
transition  period,  between  the  Carboniferous  and 
the  early  Mesozoic,  that  the  modern  cycads  and 
conifers  first  became  prominent.  These  plants  are 
most  of  them  more  or  less  xerophytic,  and  the  in- 
creased aridity  of  the  climate  of  this  period  may 
very  well  have  been  the  cause  of  the  ascendency  of 
these  plants  over  the  moisture-loving  pteridophytes 
of  the  preceding  geological  epoch. 

The  Highest  Types  of  Plants  Arose  in  the  Meso- 
zoic.— The  second  great  geological  epoch,  the  Meso- 
zoic, is  supposed  to  have  been  of  much  briefer  dura- 
tion than  the  Paleozoic,  but  it  is  noteworthy  as  the 
time  in  which  the  highest  groups  of  plants  and  ani- 
mals came  into  existence.  Birds  and  mammals,  on 
the  one  hand,  and  the  angiospermous  flowering 
plants  on  the  other,  made  their  appearance  during 
the  Mesozoic.  The  warm  but  dry  climate  of  the 
early  Mesozoic  seems  to  have  been  especially  favora- 
ble to  the  cycads,  which  at  that  period  reached  their 
culmination,  giving  place  later  to  the  more  modern 
conifers  and  angiosperms.  During  the  latter  part 


The  Problems  of  Plant  Distribution    241 

of  the  Mesozoic,  especially  the  Cretaceous,  the  cli- 
matic conditions  were  apparently  less  uniform  than 
at  the  beginning,  and  zonal  climates  were  already 
indicated,  although  much  less  pronounced  than  at 
present. 

Quite  suddenly  toward  the  end  of  the  Mesozoic, 
in  the  Sub-Cretaceous,  the  angiosperms,  the 
highest  of  all  plants,  first  appear.  Their  origin 
is  very  obscure,  but  once  developed,  they  show  an 
extraordinary  power  of  adaptation,  and  soon  out- 
number all  the  other  plants,  increasing  their  su- 
premacy until  now  they  are  by  far  the  most  impor- 
tant of  living  plants.  The  earliest  seed-bearing 
plants,  the  Cordaitales  and  Pteridosperms,  became 
extinct  towards  the  end  of  the  Paleozoic,  and  were 
replaced  by  other  types  which  have  persisted  down 
to  the  present  time.  The  Cycads,  Ginkgoales,  and 
the  lowest  Conifers  were  probably  all  in  existence 
before  the  end  of  the  Paleozoic. 

During  the  whole  of  the  Mesozoic,  North  Amer- 
ica was  connected  with  the  Eurasian  Continent,  and 
although  this  connection  was  probably  broken  down 
during  the  Tertiary,  it  was  reestablished  from  time 
to  time,  so  that  there  was  free  intermingling  of  the 
floras  throughout  the  whole  extent  of  the  Northern 
Hemisphere,  and  this  flora  maintained  its  similarity 
up  to  the  end  of  the  Tertiary. 

Cretaceous  Plants. — A  good  many  existing  gen- 
era occur  in  the  Cretaceous,  and  it  would  seem  from 
their  distribution  that  the  climate  of  the  Northern 


242  Plant  Life  and  Evolution 

Hemisphere  was  still  warm,  and  more  uniform  than 
at  the  present  time.  Many  familiar  modern  genera 
flourished,  and  while  these  Cretaceous  fossils  are 
almost  entirely  trees  or  shrubs,  it  is  only  reasonable 
to  suppose  that  many  widespread  herbaceous  genera, 
like  Ranunculus  and  Geranium,  also  existed  at  the 
same  time.  Among  the  early  Cretaceous  types  may 
be  mentioned  poplars,  willows,  and  planes.  Cer- 
tain genera,  which  now  are  represented  by  isolated 
species  in  remote  regions,  were  at  that  time  wide- 
spread; such  for  example  are  the  tulip-tree  (Lirio- 
dendron),  Magnolia,  and  Sequoia,  which  once  oc- 
curred throughout  much  of  the  Northern  Hemi- 
sphere. These  survivors  of  the  late  Mesozoic  and 
early  Tertiary  floras  are  at  present  mostly  confined 
to  warm-temperate  regions,  such  as  Japan,  the 
warmer  Atlantic  States,  and  the  mountains  of  Cali- 
fornia, and  suggest  that  the  climates  of  these  regions 
represent  approximately  the  climate  of  that  period. 
The  number  of  angiosperms  rapidly  increases 
during  the  Tertiary,  and  very  many  of  our  common 
genera  of  trees  and  shrubs  are  clearly  recogniza- 
ble. Oaks,  maples,  walnuts,  sassafras,  and  other 
familiar  forms,  all  very  much  like  existing  species, 
are  met  with,  and  we  may  suppose  that  many  of 
the  common  herbaceous  types,  which  now  accom- 
pany these,  were  also  in  existence,  although  owing 
to  their  perishable  nature  they  have  left  no  recog- 
nizable fossil  remains. 


The  Problems  of  Plant  Distribution    243 

FACTORS  CONCERNED  IN  PLANT  DISTRIBUTION 

The  factors  that  have  been  active  in  determining 
the  distribution  of  the  existing  floras  of  the  earth 
are  many  and  complicated.  Some  of  them  are  suf- 
ficiently clear,  but  of  many  we  are  quite  ignorant  at 
present.  Of  the  most  obvious  conditions,  probably 
climate,  i.e.,  temperature  and  moisture,  is  of  most 
importance;  but  several  other  very  evident  factors 
may  be  mentioned  which  play  parts  quite  as  impor- 
tant in  the  distribution  of  plants.  These  are  the 
continuity  of  land  areas,  composition  of  the  soil,  and 
the  exposure  of  the  soil.  But  perhaps  most  impor- 
tant of  all  are  the  individual  characters  of  the  plants 
concerned,  some  being  especially  adaptable  and  pro- 
vided with  ready  means  of  transportation,  others 
very  particular  as  to  their  requirements  of  growth, 
and  therefore  confined  to  extremely  limited  areas. 
It  is  the  underlying  causes  of  these  great  differences 
in  the  very  constitution  of  different  plants,  that 
are  the  most  obscure  and  little  understood  factors 
governing  plant  distribution. 

Uniform  Flora  of  Northeastern  United  States — 
Where  there  are  extensive  areas  with  uniform  cli- 
matic conditions,  and  no  barriers  to  prevent  ready 
communication,  the  flora  will  be  found  to  be  very 
similar,  varying  only  with  the  local  peculiarities  of 
soil  or  elevation.  The  Northeastern  United  States 
is  an  excellent  example  of  such  an  area.  A  very 
large  number  of  species  occur  throughout  the  entire 


244  Plant  Life  and  Evolution 

region,  and  the  greater  richness  or  poverty  of 
species  in  different  localities  is  due  only  to  local  con- 
ditions. The  prevailing  trees,  pines,  hemlocks, 
oaks,  maples,  elms,  hickories,  beeches,  etc.,  are 
the  same  everywhere,  the  flora  naturally  being  richer 
in  the  warmer  and  moister  southern  portions  than  in 
the  colder  and  drier  northwest.  The  shrubs  and 
herbaceous  plants  are  much  the  same  throughout,  of 
course  taking  into  account  the  local  differences  of 
soil  and  exposure. 

Effect  of  Varying  Rainfall  in  Jamaica. — The  gen- 
eral uniformity  of  such  a  flora  as  that  just  sketched 
has  only  to  be  contrasted  with  the  flora  of  a  very 
much  smaller  area,  where  for  special  reasons  ad- 
jacent districts  differ  much  in  climate,  especially  in 
the  amount  of  rainfall.  In  Jamaica,  for  instance, 
within  a  distance  of  about  forty  miles,  mountains 
rise  to  a  height  of  over  7,000  feet,  and  cause  the  pre- 
cipitation of  most  of  the  moisture  upon  one  side  of 
the  range,  the  northern  side  receiving  from  three  to 
four  times  as  much  rain  as  the  southern  side  does, 
only  forty  miles  away.  The  result  is  that  the  vege- 
tation of  these  two  areas  is  more  different  than  that 
of  Chicago  and  New  York,  nearly  a  thousand  miles 
apart. 

At  Kingston,  on  the  southern  shore  of  the 
island,  the  dry  plains  and  hillsides  recall  our  south- 
western arid  region,  the  prevailing  plants  being 
decidedly  xerophytic  in  character.  Thus  cacti,  cen- 
tury-plants, mesquit,  and  many  other  plants,  belong- 


The  Problems  of  Plant  Distribution    245 

ing  to  a  distinctly  arid  region,  are  the  conspicuous 
features  of  the  flora.  At  Port  Antonio,  on  the 
northern  shore,  there  is  a  veritable  tropical  jungle; 
the  trees  are  laden  down  with  heavy  creepers,  and 
dense  masses  of  epiphytes,  and  aroids,  wild  ginger, 
palms,  peppers,  bananas,  and  many  other  repre- 
sentatives of  the  wet  tropics,  crowd  the  spaces  be- 
tween the  creeper-laden  trees.  Of  course  in  a  trop- 
ical region  the  differences  would  be  much  more 
marked  than  in  a  temperate  one,  where  the 
conditions  for  plant  growth  are  so  much  less 
intense. 

The  amount  of  moisture  and  the  character  of  the 
soil  have  very  much  to  do  with  determining  the 
vegetation  of  any  area.  The  differences  between 
the  flora  of  a  swamp,  and  that  of  a  dry  hillside  in 
the  immediate  neighborhood,  are  complete,  and 
probably  no  single  plant  will  be  common  to  both. 
The  floras  of  peat  bogs  are  almost  always  exception- 
ally peculiar.  While  many  plants  are  quite  unable  to 
live  in  these,  there  are  others  which  have  adapted 
themselves  to  the  very  peculiar  conditions  of  the  bog 
and  refuse  to  grow  elsewhere.  Some  of  the  most 
beautiful  orchids,  the  pitcher-plants,  and  sundews 
belong  to  this  category. 

Character  of  Soil  a  Factor  in  Distribution. — The 
question  of  soil  is  a  very  complicated  one  and  in- 
volves numerous  factors.  Many  plants  are  exceed- 
ingly sensitive  to  the  character  of  the  soil  in  which 
they  grow.  Thus  many  of  the  Heath  family, 


246  Plant  Life  and  Evolution 

Azaleas  and  Rhododendrons,  avoid  soils  containing 
lime,  while  for  many  other  plants  lime  is  an  essen- 
tial. The  mechanical  characters  of  the  soil,  that  is, 
whether  it  is  compact  or  loose,  retentive  of  water 
or  the  reverse,  are  also  important  factors  in  deter- 
mining the  distribution  of  many  plants.  Finally 
the  special  devices,  like  winged  seeds  and  fruits, 
play  an  important  part  in  determining  the  distribu- 
tion of  some  plants.  Most  weeds  are  such  because 
of  the  facility  with  which  they  can  be  distributed. 
With  this  facility  for  distribution  there  also  goes 
the  hardiness  and  adaptability  which  these  plants 
exhibit.  One  has  but  to  contrast  the  dandelion  with 
such  an  orchid  as  Arethusa,  for  example,  to  realize 
the  difference  between  a  really  adaptable  plant  and 
an  exceptionally  particular  one. 

Ancient  Distribution  of  Land. — There  are  still 
evident  some  traces  of  the  ancient  divisions  of  the 
land  areas  of  the  world  into  a  northern  and  south- 
ern mass,  shown  by  the  character  of  the  vegetation. 
A  good  many  families  of  plants  are  still  confined 
respectively  to  the  Northern  and  Southern  Hemi- 
sphere. Thus  many  conifers,  the  pines,  firs,  etc.,  are 
distinctly  northern  types.  The  Araucarias  and 
Kauri  pine  (Agathis)  are  equally  characteristic 
southern  coniferous  types.  Among  the  angiosperms, 
willows,  oaks,  birches,  and  maples  are  examples  of 
families  practically  confined  to  the  Northern  Hemi- 
sphere. The  very  peculiar  Casuarina,  sometimes 
cultivated  in  California,  and  the  Proteaceae,  of 


The  Problems  of  Plant  Distribution    247 

which  the  silk-oak  (Grevillea)   is  the  best  known, 
are  examples  of  characteristic  austral  families. 

Floras  of  the  Old  and  New  World. — The  main 
elements  of  the  north-temperate  floras,  of  both  the 
Old  and  the  New  World,  are  evidently  derived  from 
the  Tertiary  flora  of  the  ancient  northern  continent, 
and  many  families  and  genera  are  still  common  to 
the  Eurasian  continent  and  North  America.  In 
the  tropics,  as  we  have  already  pointed  out,  the  dif- 
ferences between  the  Old  and  New  World  are  very 
marked.  Probably  the  two  richest  botanical  regions 
in  the  world  are  the  Indo-Malayan  region  and 
tropical  South  America.  A  comparison  of  these  two 
regions  shows  very  few  genera  in  common,  and 
there  are  even  many  families  which  are  peculiar 
to  one  region  or  the  other.  For  instance,  the  palms 
of  the  Old  and  New  World  belong  almost  without 
exception  to  different  genera,  and  the  same  is  true 
of  the  vast  majority  of  the  orchids  and  other  large 
families.  The  whole  family  of  the  Screw-pines  is 
confined  to  the  Old  World,  and  the  no  less  marked 
Pineapple  family  is  peculiar  to  America.  Where 
there  are  genera  common  to  the  tropics  of  both 
hemispheres,  they  are  usually  widespread  ones,  with 
representatives  in  the  temperate  zones  between,  and 
usually  they  are  genera  provided  with  very  favora- 
ble means  of  distribution,  such  as  certain  Compositae 
like  Vernonia  and  Senecio.  The  great  differences 
in  the  character  of  the  floras  of  the  two  great  trop- 
ical regions  are  easily  understood,  since  these  are 


248  Plant  Life  and  Evolution 

so  very  much  isolated,  and  under  the  forcing  condi- 
tions of  the  tropics,  and  the  sharp  struggle  for  ex- 
istence, the  change  in  species  is  presumably  much 
more  rapid  than  is  the  case  in  the  temperate  zones. 

Present  Conditions  in  the  Southern  Hemisphere. 
— The  conditions  in  the  Southern  Hemisphere,  at 
present,  are  very  different  from  those  in  the  North. 
The  Antarctic  continent  is  an  absolutely  barren 
waste,  with  scarcely  a  vestige  of  any  vegetation,  and 
it  is  separated  completely  from  the  three  principal 
land  masses  of  the  South — Australia,  South  Amer- 
ica, and  South  Africa.  While  in  the  course  of  the 
ages  which  have  elapsed  since  the  three  latter  were 
united,  the  vegetation  has  become  very  much  al- 
tered, there  still  are  evidences  of  a  common  origin 
for  the  floras,  although  this  is  by  no  means  so 
marked  as  in  the  Northern  Hemisphere.  The  Arau- 
carias  of  South  America  and  Australia,  and  the  Pro- 
teaceae  found  in  all  three  regions,  are  presumably 
descendants  of  the  common  primordial  flora  of  the 
ancient  southern  continent. 

Floras  of  Isolated  Regions. — Wherever  a  region 
is  shut  off  by  barriers,  either  mountains,  desert,  or 
sea,  the  flora  is  certain  to  be  very  peculiar.  In  such 
isolated  regions  as  the  Cape  region  of  Africa,  West- 
ern Australia,  or  even  regions  like  California  or 
the  shores  of  the  Mediterranean,  this  is  very  evi- 
dent. In  all  of  these,  climatic  conditions  are  more 
or  less  similar  and  all  of  them  have  developed  very 
rich  and  peculiar  floras  that  show  some  interesting 


The  Problems  of  Plant  Distribution    249 

analogies  in  the  plants,  although  these  may  not  be 
at  all  closely  related.  For  example,  both  the  Cape 
and  California  are  characterized  by  a  very  large 
number  of  showy  bulbous  plants,  but  those  in  Cali- 
fornia are  mostly  of  the  Lily  family,  while  in  South 
Africa  it  is  the  Iris  family  which  is  especially  de- 
veloped. The  cacti  and  century-plants  of  our 
Southwest  are  replaced  in  the  drier  parts  of  Africa 
by  the  leafless  Euphorbias  and  Aloes,  which  super- 
ficially resemble  to  a  remarkable  degree  the  Amer- 
ican cacti  and  agaves,  but  are  not  at  all  closely  re- 
lated to  them  botanically.  It  is  probable  that  most 
of  the  existing  plant  types  were  pretty  well  differ- 
entiated in  the  later  Tertiary  and,  as  the  fossil  rec- 
ords show,  the  flora  was  fairly  uniform  over  the 
Northern  Hemisphere.  At  that  period  there  is  evi- 
dence that  many  existing  genera,  which  are  now 
restricted  in  their  range,  were  widespread.  In  Eu- 
rope, and  even  in  Siberia  and  Greenland,  there  are 
found  remains  of  such  genera  as  Sequoia,  Tax- 
odium,  Liriodendron,  Magnolia,  Sassafras;  and 
laurels,  and  even  palms  abounded,  all  of  which 
have  long  since  vanished  from  these  regions,  but 
whose  descendants  still  flourish  in  some  more  or 
less  isolated  regions,  where  they  have  survived  the 
great  readjustment  of  the  flora,  resulting  from  the 
Glacial  epoch. 

At  the  present  day  we  may  recognize  a  sub-polar 
zone,  north  and  south  temperate,  and  tropical  zones, 
which  of  course  are  not  absolutely  denned.  The 


250  Plant  Life  and  Evolution 

vegetation  of  the  south  polar  zone  is  so  scant  as  to 
practically  amount  to  nothing. 

The  Sub-polar  Zone. — In  the  sub-polar  zone 
generally,  much  the  same  conditions,  except  tem- 
perature, prevail  as  were  found  during  the  pre- 
glacial  epoch,  and  there  is  the  same  uniformity 
of  vegetation,  which,  however,  is  much  scantier, 
as  might  be  expected  from  the  more  rigorous  cli- 
matic conditions  which  now  prevail.  Throughout 
this  area,  from  Northern  Scotland  and  Scandinavia, 
to  Eastern  Canada,  the  same  types  give  character  to 
the  vegetation.  Poplars,  willows,  firs,  and  birches 
are  the  predominant  trees,  and  in  the  mead- 
ows and  bogs  many  beautiful  flowering  herbaceous 
plants  give  a  charm  to  the  brief  summers  of  these 
high  northern  regions.  Of  course  there  are  many 
plants  in  Canada  and  Alaska  which  do  not  occur  in 
the  Old  World,  but  these  are  mostly  emigrants  from 
the  South  and  may  be  said  not  properly  to  belong 
to  the  sub-polar  zone. 

The  North  Temperate  Zone. — Proceeding  south- 
ward from  this  uniform  northern  zone  of  vegeta- 
tion, the  increasing  warmth  causes  a  corresponding 
greater  diversity  in  the  vegetation,  this  diversity 
becoming  more  and  more  marked  as  the  warmer 
tropical  zones  are  approached.  As  the  temperate 
zones  of  the  Old  and  New  World  are  now  com- 
pletely isolated,  and  have  been  so  since  the  close  of 
the  Glacial  epoch,  a  very  much  greater  difference 
between  the  floras  of  the  Old  and  New  World  is 


The  Problems  of  Plant  Distribution    251 

found  than  is  the  case  in  the  northern  regions. 
Even  where  common  genera  occur,  there  has  been 
the  development  of  new  species,  owing  to  climatic 
differences,  and  very  few  species  are  really  common 
to  these  two  regions  except  where  these  species  also 
occur  in  the  northern  zone.  Thus  while  Europe  and 
Atlantic  North  America  possess  many  genera  in 
common,  like  the  oaks,  elms,  walnuts,  larches, 
asters,  goldenrods,  gentians,  violets,  etc.,  they  are 
with  few  exceptions  represented  by  quite  distinct 
species.  All  of  these  may  safely  be  considered  to 
be  the  common  descendants  of  Tertiary  ancestors, 
which  through  isolation  have  become  specifically  dis- 
tinct. On  the  other  hand,  there  are  numerous  types 
which  belong  to  one  or  the  other  of  the  two  regions 
but  are  absent  from  the  other.  Thus  Europe  has 
no  magnolias,  tulip-trees,  gums,  sassafras,  hick- 
ories, trilliums,  milk-weeds,  mandrakes,  and  very 
many  more  familiar  American  plants;  while  on  the 
other  hand,  America  possesses  no  daffodils,  tulips, 
snowdrops,  foxgloves,  heaths,  brooms,  and  many 
other  beautiful  flowers,  which  adorn  the  woods 
and  meadows  of  the  Old  World. 

Within  the  north  temperate  zone  are  enormous 
areas  showing  far  greater  differences  of  conditions 
than  are  found  in  the  regions  of  the  North,  and  in 
consequence  their  floras  are  far  more  varied.  In  the 
southern  portions  of  this  zone  there  is  frequently  an 
invasion  of  tropical  types,  and  the  limits  between 
the  temperate  and  tropical  floras  are  very  vague. 


252  Plant  Life  and  Evolution 

The  Tropics. — With  the  approach  to  the  tropics 
the  northern  types  of  vegetation  gradually  disap- 
pear, and  are  replaced  by  quite  new  ones.  It  is  true 
that  such  northern  types  as  pines  and  oaks  may 
invade  the  tropics,  but  these  are  exceptional,  and  for 
the  most  part  the  plants  of  the  hotter  regions  of 
the  world  are  members  of  genera,  and  often  of 
families,  not  represented  at  all  in  the  colder  parts 
of  the  world.  The  more  intense  growth  conditions 
and  the  fierce  struggle  for  existence  result  in  a 
great  diversity  of  plant-types  adapted  to  all  condi- 
tions of  existence.  It  is  in  the  tropics  that  one 
fully  appreciates  the  possibilities  of  plant  adaptation. 
Every  tree  in  a  tropical  jungle  is  a  veritable  botan- 
ical garden,  its  trunk  and  branches  covered  with  a 
mass  of  epiphytic  growths,  and  giant  creepers  often 
overtop  its  highest  branches.  With  this  luxuriant 
growth  there  has  developed  an  almost  infinite  va- 
riety of  forms  adapted  to  quite  special  conditions, 
and  the  differences  between  the  plant  types  of  the 
tropics  of  the  Old  and  New  Worlds  are,  as  we  have 
seen,  far  greater  than  is  the  case  in  the  temperate 
zones. 

It  is  only  among  the  older  and  more  conserva- 
tive types  of  vegetation  that  the  same  or  closely 
allied  species  occur.  Thus  while  among  the  algae, 
mosses,  and  ferns  there  are  very  many  genera,  or 
even  species,  that  are  common  to  the  tropics  of  both 
hemispheres,  among  the  flowering  plants  it  is  ex- 
ceptional to  find  any  genera  in  common,  and  where 


The  Problems  of  Plant  Distribution    253 

they  do  occur,  they  are,  as  we  have  seen,  genera 
which  are  widespread  throughout  the  temperate 
regions.  Senecio,  Vernonia,  Acacia,  and  Ipomoea 
are  examples  of  some  of  these  widespread  genera. 
Among  the  characteristic  tropical  types,  like  the 
palms,  aroids,  bananas,  etc.,  very  few  genera,  even, 
are  common  to  both  hemispheres.  Sometimes  an 
Old  World  family  is  represented  by  an  allied  one  in 
the  tropics  of  the  New  World.  Thus,  for  instance, 
the  Ginger  family  is  only  found  in  the  Old  World, 
the  Canna  family  in  the  new. 

The  South  Temperate  Zone.— The  south  tem- 
perate regions  are  very  much  more  isolated  than 
those  of  the  north,  and  for  the  most  part  have 
very  different  types  of  vegetation,  there  being  very 
little  in  common,  for  instance,  between  the  flora 
of  Argentina  and  that  of  South  Africa.  There  are, 
however,  certain  similarities  between  the  flora  of 
the  latter  and  some  of  the  more  temperate  parts 
of  Australia,  and  this  is  true  also  of  some  parts 
of  South  America,  as,  for  example,  the  occurrence 
of  Proteaceae  and  Araucaria,  which  point  to  an  an- 
cient connection  between  these  southern  regions  and 
denote  that  the  flora  of  all  of  these  regions  have 
had  a  common,  but  very  remote,  origin.  Unfortu- 
nately our  knowledge  of  the  fossils  of  the  Southern 
Hemisphere  is  very  incomplete,  and  for  the  present 
the  geological  history  of  the  flora  must  remain  un- 
satisfactory. 

Cretaceous    and    Tertiary    Plants. — There    are 


254  Plant  Life  and  Evolution 

abundant  plant  remains  from  Cretaceous  and  Ter- 
tiary deposits  throughout  most  of  the  Northern 
Hemisphere,  which  give  a  very  good  idea  of  the 
character  of  the  vegetation  of  those  periods.  The 
fossils  are  largely  impressions  of  leaves,  mainly  of 
trees  and  shrubs,  the  more  delicate  herbaceous  vege- 
tation having  left  no  traces.  The  leaf  impressions 
are  often  exceedingly  perfect,  and  in  many  cases 
quite  unmistakable,  and  it  is  evident  from  a  study 
of  these  fossils  that  many  modern  genera  were  well 
represented.  Oaks,  poplars,  willows,  planes,  com- 
mon northern  types  of  the  present  day,  were  com- 
mon and  widespread,  and  with  these,  very  often 
in  localities  now  quite  unfitted  for  their  growth, 
were  genera  belonging  to  warm  climates,  like  the 
magnolias,  palms,  and  laurels.  The  conclusion 
has  been  drawn  that  during  the  early  Tertiary  there 
was  a  fairly  uniform  flora  throughout  what  is  now 
the  north  temperate  and  sub-polar  regions,  but  that 
the  climate  was  much  warmer  than  that  now  pre- 
vailing in  the  northern  regions.  While  we  have 
little  knowledge  from  the  fossil  record  of  the  her- 
baceous plants  accompanying  the  trees  and  shrubs 
whose  remains  occur  in  Tertiary  deposits,  a  study 
of  the  distribution  of  the  living  species  gives  us 
some  clue  as  to  what  many  of  these  probably  were. 
Such  widespread  types  as  buttercups,  anemones, 
violets,  lilies,  and  many  other  familiar  flowers, 
were  in  all  probability  represented  by  species  not 
very  different  from  their  living  descendants,  and 


The  Problems  of  Plant  Distribution    255 

there  were  probably  others  which  at  present  are 
restricted  to  limited  areas,  but  which  were  more 
widely  distributed  during  the  Tertiary. 

By  the  end  of  the  Tertiary  there  was  an  evident 
lowering  of  the  temperature,  and  the  zonal  climates 
were  already  well  marked,  but  less  pronounced  than 
at  present.  Toward  the  end  of  the  Tertiary  the 
general  distribution  of  the  land  areas  was  much 
as  at  present  and  the  land  connection  between  Eu- 
rope and  America  was  permanently  severed.  The 
end  of  the  Tertiary  was  followed  by  the  gradual 
formation  of  the  great  ice-sheet,  inaugurating  the 
Glacial  epoch.  The  great  climatic  disturbances 
due  to  the  development  of  the  great  polar  ice-sheet, 
resulted  in  very  marked  changes  in  the  distribution 
of  the  uniform  northern  Tertiary  flora,  and  the  ad- 
vance of  the  ice-sheet  was  the  principal  factor  in 
determining  the  distribution  -of  the  present  flora 
of  the  Northern  Hemisphere.  With  the  increasing 
cold,  and  the  southward  extension  of  the  great 
glaciers,  vegetation  of  all  kinds  must  have  been 
forced  southward.  The  result  of  this  was  very  dif- 
ferent in  different  parts  of  the  world.  In  Europe, 
which  lies  mostly  within  the  region  of  severe  glacia- 
tion  and  whose  great  mountain  ranges  formed  bar- 
riers against  the  southward  retreat  of  the  more 
tender  plants,  many  plants  were  destroyed  which 
have  survived  under  the  more  favorable  conditions 
presented  in  Eastern  Asia  and  America. 

It  is  evident  from  a  study  of  European  Cretaceous 


256  Plant  Life  and  Evolution 

and  Tertiary  fossils,  that  many  genera  once  grew 
freely  there,  which  are  now  quite  extinct,  but  which 
have  survived  in  America  and  Asia.  Among  these 
were  cypresses,  closely  allied  to  our  southern  bald- 
cypress;  Sequoias,  related  to  the  California  big-trees, 
and  redwoods;  hickories,  sassafras,  tulip-trees, 
magnolias,  gums,  and  other  familiar  denizens  of 
our  American  forests.  Some  of  these  genera  still 
survive  in  Eastern  Asia,  where  conditions  during 
the  period  of  glaciation  were  quite  like  those  in 
America,  and  where  the  present  climatic  conditions 
are  also  very  much  the  same.  In  both  Eastern 
Asia  and  America  there  is  a  continuous  land  ex- 
tension southward,  and  the  mountains  run  north 
and  south,  so  that  no  barriers  prevented  the  retreat 
of  the  vegetation  before  the  encroaching  glaciers, 
and  the  plants  returned  northward  as  the  glaciers 
receded. 

Similarity  in  Floras  of  Eastern  Asia  and  Eastern 
America. — The  great  similarity  in  the  general  char- 
acter of  the  floras  of  the  Manchurian  and  Japanese 
regions,  and  to  some  extent  that  of  China  and  the 
Himalayas,  and  that  of  Atlantic  North  America,  is 
most  marked.  This  is  especially  seen  in  the  occur- 
rence of  certain  small  peculiar  genera  with  no  repre- 
sentatives in  the  intervening  countries.  This  sub- 
ject was  one  to  which  Professor  Asa  Gray  gave 
much  attention,  and  his  work  is  of  very  great  value 
and  interest.  He  cites  a  long  list  of  genera  common 
to  these  two  regions,  but  absent  from  the  regions- 


The  Problems  of  Plant  Distribution    257 

between.  Only  a  few  cases  can  be  given  here,  but 
these  will  be  sufficient  to  illustrate  the  point.  The 
very  characteristic  tulip-tree  (Liriodendron)  of  At- 
lantic North  America  has  an  almost  identically  sim- 
ilar species  occurring  in  China ;  the  genus  Magnolia 
belongs  solely  to  Eastern  Asia  and  Eastern  Amer- 
ica; Wistaria,  Stuartia,  Ampelopsis,  Hamamelis, 
and  many  others  show  a  like  distribution.  Many  of 
these  occur  fossil,  showing  that  they  were  once 
widespread,  and  that  their  present  occurrence  is  a 
case  of  survival  in  widely  separated  regions  where 
conditions  happened  to  be  favorable.  While  we 
know  from  the  fossil  records  that  these  isolated 
types  of  trees  and  shrubs  were  once  widespread, 
we  can  only  conjecture  that  the  same  was  true  of 
certain  plants  whose  distribution  is  now  similar,  but 
of  which  we  have  no  fossil  record.  Among  the 
most  peculiar  plants  of  Atlantic  North  America 
are  certain  herbaceous  plants  of  the  Barberry  fam- 
ily. The  mandrake  (Podophyllum),  and  the  twin- 
leaf  (Jeffersonia),  are  examples  of  these  isolated 
types.  Each  of  these  is  represented  in  Eastern 
America  by  a  single  species,  and  the  occurrence  of 
another  closely  allied  species  in  such  remote  regions 
as  Japan  and  the  Himalayas,  makes  it  almost  certain 
that  these  must  be  Tertiary  genera  once  widespread, 
which  have  survived,  just  as  the  tulip-trees  and 
magnolias  have  done  in  specially  favored  places.  A 
long  list  of  others  might  be  cited,  but  one  more  must 
suffice.  The  beautiful  trailing  arbutus,  or  May- 


258  Plant  Life  and  Evolution 

flower,  of  our  Eastern  woods,  has  its  mate  in  a 
second  species  growing  in  Japan,  while  elsewhere 
the  genus  is  quite  unknown. 

As  might  be  expected  in  the  ages  that  have  elapsed 
since  the  redistribution  of  the  Tertiary  flora  took 
place,  most  of  the  forms  have  changed  to  some  ex- 
tent, so  that  it  is  rare  to  meet  identical  species  in 
such  widely  separated  regions  as  Japan  and  New 
England.  The  change  has  gone  so  far  in  some 
cases  that  a  genus  of  one  district  is  represented  by 
a  different  but  closely  allied  one  in  the  other.  Thus, 
for  example,  the  flowering  dogwood  of  Eastern 
America  is  represented  in  Japan  by  a  closely  allied 
genus,  Benthamia.  Identical  species,  however,  may 
occur.  The  poison-ivy  and  the  fox-grape  of  At- 
lantic North  America  are  represented  in  Japan  by 
what  are  usually  considered  to  be  identical  species, 
and  the  sensitive-fern  and  the  beautiful  little 
orchid,  Pogonia,  are  the  same  in  Japan  and  Massa- 
chusetts. 

Our  knowledge  of  the  geological  history  of  the 
flora  of  the  tropics  is  still  incomplete,  and  as  these 
regions  were  not  influenced  materially  by  the  great 
Glacial  epoch,  and  as  the  conditions  in  the  tropical 
regions  are  conducive  to  rapid  evolution  of  new 
forms,  it  is  not  remarkable  that  the  tropical  floras 
should  differ  very  widely  from  the  temperate  ones. 
Moreover,  as  the  tropics  of  the  Old  and  New  World 
must  have  been  completely  isolated  from  very  re- 
mote times,  migration  from  one  to  the  other,  ex- 


The  Problems  of  Plant  Distribution    259 

cept  in  modern  times,  must  have  been  so  infrequent 
as  to  be  practically  of  no  account,  and  this  explains 
the  rarity  even  of  generic  types  common  to  them. 
Except  where  these  have  been  introduced  by  man, 
they  are,  as  we  have  seen,  genera  which  are  wide- 
spread and  generally  occurring  also  in  the  temperate 
zones. 

The  development  of  the  great  continental  areas  in 
Asia  and  America  caused  pronounced  changes  in 
the  climate,  and  the  inner  areas,  arid  and  subjected 
to  extremes  of  burning  heat  and  arctic  cold,  could 
no  longer  support  the  moisture-loving  plants  which 
prevailed  in  pre-glacial  times,  and  hence  the  restric- 
tion of  these  to  the  moister  and  more  temperate 
regions  nearer  the  coast.  These,  arid  plains  also 
acted  as  a  barrier  against  migration  to  the  regions 
of  the  West,  which  were  better  adapted  to  their 
growth.  In  the  United  States,  however,  the  Pacific 
coast,  owing  perhaps  to  its  long  dry  summers,  is 
not  at  present  suited  to  the  growth  of  many  eastern 
trees,  although  we  know  that  some  of  these  once 
existed  there. 

Influence  of  the  Western  Mountains  upon  the 
Climate  of  the  United  States. — In  the  United 
States,  the  development  of  the  great  mountain 
masses  of  the  West,  must  have  exerted  a  great 
influence  in  determining  the  climate  of  the 
great  central  area,  by  shutting  off  the  moisture- 
laden  winds  of  the  Pacific.  The  Rocky  Mountains 
were  formed  at  the  end  of  the  Cretaceous, 


26o  Plant  Life  and  Evolution 

and  presumably  the  region  immediately  to  the 
east  of  the  mountains  then,  as  now,  was  one  of 
slight  rainfall.  In  a  recent,  very  interesting  study 
of  the  prairie  flora,  Harvey  has  given  a  very 
plausible  explanation  of  the  divergencies  of  the 
western  and  eastern  floras  within  the  United  States. 
He  holds  that  even  during  the  Tertiary  the  great 
plains  region  was  too  dry  for  the  growth  of  forests. 
With  the  retreat  of  the  northern  forests  before  the 
advancing  glaciers,  these  prairie  regions,  unfitted  for 
forest  growth,  acted  as  a  wedge,  one  company  of 
migrants  working  to  the  westward,  and  character- 
ized by  the  predominance  of  coniferous  trees;  the 
other  flowing  eastward,  and  typically  deciduous,  fol- 
lowed the  Mississippi  and  its  tributaries  and  became 
settled  in  the  Appalachian  region  of  North  Carolina 
and  Tennessee,  where  to-day  it  forms  the  finest  de- 
ciduous forests  in  our  country. 

ALPINE  PLANTS 

Very  interesting  is  the  survival  of  many  northern 
plants  on  high  mountains,  often  very  remote  from 
each  other.  It  is  supposed  that  these  northern 
plants,  driven  southward  by  the  increasing  cold,  re- 
treated up  to  the  cooler  regions  of  the  higher  alti- 
tudes as  the  climate  became  warmer,  after  the  re- 
treat of  the  glaciers.  Even  in  the  tropics,  close  to 
the  equator,  one  meets  on  the  tops  of  high  moun- 
tains a  real  northern  flora,  including  such  familiar 


The  Problems  of  Plant  Distribution    261 

types  as  buttercups,  strawberries,  violets,  brambles, 
primroses,  gentians,  and  others  obviously  of  north- 
ern origin,  and  quite  unrelated  to  any  of  the  plants 
of  the  adjacent  lowlands.  These  isolated  waifs,  in 
the  course  of  ages,  and  under  the  milder  conditions 
prevailing,  even  on  the  high  mountains  in  the  tropics, 
have  become  specifically  changed,  but  have  neverthe- 
less retained  their  generic  characters. 

Some  of  the  high,  isolated  peaks  of  the  tropics 
afford  striking  instances  of  the  change  in  vegeta- 
tion due  to  altitude.  The  great  volcanic  mass  of 
the  Gedeh  in  Western  Java,  is  an  especially  good 
example  of  this.  This  mountain  rises  to  a  height 
of  10,000  feet,  and  lies  but  a  few  degrees  from 
the  equator  in  a  region  of  very  heavy  rainfall.  As 
one  ascends  from  the  luxuriant  tropical  vegetation 
of  the  lowlands  at  the  base  of  the  mountain,  a  change 
is  very  soon  apparent.  At  about  4,500  feet  (1,400 
m.)  the  temperature  has  fallen  many  degrees,  and 
although  the  heavy  rains  and  almost  constant  clouds 
and  mist  promote  an  extraordinarily  luxuriant  for- 
est growth,  many  of  the  strictly  tropical  types  like 
palms  and  bamboos  have  nearly  disappeared,  and  a 
number  of  northern  types  become  common.  Oaks, 
chestnuts,  and  maples  occur,  and  several  trees  be- 
longing to  genera  common  in  our  Southeastern 
States  are  met  with.  The  loftiest  tree  of  this  moun- 
tain forest  (Altingia  cxcelsd)  is  related  to  the  sweet 
gum  (Liquidambar)  of  the  Eastern  United  States; 
and  species  of  Nyssa,  like  the  pepperidge  or  "  sour 


262  Plant  Life  and  Evolution 

gum,"  of  Eastern  America,  also  occur,  and  one  of 
the  showiest  of  the  smaller  trees,  Gordonia,  with 
big  white  flowers  like  Cherokee  roses,  belongs  to 
the  same  genus  as  the  loblolly  bay  of  the  Gulf  States. 
The  northern  aspect  of  the  vegetation  increases 
rapidly  as  the  summit  of  the  mountain  is  ap- 
proached. The  low,  gnarled  trees  composing  the 
forest  are  bearded  with  gray  lichens,  like  those  of 
the  northern  forests,  but  from  these  same  trees  hang 
beautiful  orchids,  not  at  all  reminiscent  of  the 
North,  and  the  stately  tree-ferns,  as  well  as  many 
other  plants  unfamiliar  to  the  northern  botanist, 
remind  him  that  he  is  still  in  the  tropics,  in  spite 
of  the  cold  gray  skies.  At  this  altitude  a  number 
of  showy  small  trees  and  shrubs  of  northern  origin 
are  common.  Huckleberries,  wintergreen  (Gaul- 
theria),  and  fine  orange  and  scarlet  rhododendrons 
are  common.  Thickets  of  brambles,  and  carpets  of 
everlastings  (Gnaphalium),  buttercups,  violets,  bal- 
sams, and  other  familiar  flowers  abound,  and  in 
the  sheltered  thickets  among  the  bushes  are  colonies 
of  a  stately  primrose,  Primula  imperialis,  which  for 
many  years  was  known  only  from  this  mountain. 
Many  of  these  plants,  like  the  primrose,  have  their 
nearest  relatives  in  the  Himalayas,  and  it  has  been 
suggested  that  the  seeds  of  some  of  them  may  have 
been  carried  by  the  strong,  prevailing  winds  of  the 
upper  atmosphere,  which  blow  southeastward  for 
long  periods.  This,  however,  seems  hardly  probable 
in  the  case  of  many  of  the  species,  whose  presence 


The  Problems  of  Plant  Distribution    263 

on  this  isolated  mountain  top  is  better  explained  on 
the  theory  of  migration  due  to  the  Glacial  epoch. 

The  Alpine  plants  of  the  temperate  regions  may 
be  specifically  the  same  as  species  growing  at  sea- 
level  in  higher  latitudes.  On  the  higher  summits 
of  the  New  England  mountains  and  the  southern 
Alleghanies,  one  sees  tufts  of  the  pretty  little  Green- 
land sandwort  (Arenaria  grcenlandica) ,  which  is  un- 
known in  the  neighboring  lowlands,  but  flourishes  at 
sea-level  in  Labrador  and  Greenland;  and  in  the 
higher  regions  of  the  Rocky  Mountains  as  far  west 
as  Utah,  a  beautiful  little  flower,  Dry  as  octopetala, 
one  of  the  most  characteristic  of  arctic  flowers,  is 
a  common  and  conspicuous  species.  This  species  is 
also  abundant  upon  the  mountain  summits  of 
Europe. 

ISLAND  FLORAS 

Remote  islands  afford  some  interesting  problems 
in  the  evolution  of  new  species.  The  more  remote 
the  island,  the  less  likely  that  new  forms  will  be 
brought  to  its  shores,  and  the  more  probable  that 
the  forms  which  do  so  will  have  time  to  change,  in 
accordance  with  the  new  conditions  to  which  they 
are  subjected.  Perhaps  the  most  striking  case 
known  is  that  of  the  Hawaiian  Islands.  These  are 
volcanic  masses,  thrown  up  from  great  depths,  and 
separated  by  long  distances  from  any  other  land. 
The  islands  are  of  different  ages,  and  evolutionary 


264  Plant  Life  and  Evolution 

forces  have  been  at  work  longer  in  some  of  them 
than  in  others,  and  it  can  be  clearly  seen  that  this 
longer  time  has  been  efficient  in  producing  a  more 
varied  and  specialized  flora  in  the  older  islands. 

Thus  the  island  of  Kauai,  the  oldest  of  the  group, 
has  many  more  peculiar  species  than  the  very  much 
larger  but  more  recent  island  Hawaii,  which  is  still 
in  process  of  formation.  The  flora  of  these  islands 
is  derived  mainly  from  the  Polynesian  region  to  the 
south,  but  there  are  also  evidences  of  some  American 
immigrants.  Most  of  these,  however,  have  become 
so  changed,  that  of  the  certainly  indigenous  species 
of  vascular  plants,  it  has  been  claimed  that  over 
eighty  per  cent  are  endemic,  i.e.,  are  peculiar  to  the 
islands,  a  proportion  probably  unequaled  in  any 
other  region  of  the  world,  except  perhaps  in  the  very 
isolated  area  of  Western  Australia. 

The  Flora  of  Krakatau. — The  reestablishment  of 
the  vegetation  upon  the  island  of  Krakatau  is  an 
instructive  demonstration  of  the  origin  of  island 
floras.  In  1883,  this  volcanic  island  lying  in  the 
Straits  of  Sunda,  between  Java  and  Sumatra,  was 
the  scene  of  the  most  violent  volcanic  disturbance 
that  has  ever  been  recorded.  The  greater  part  of 
the  island  was  blown  into  space  by  the  great  ex- 
plosion, and  what  was  left,  as  well  as  some  of  the 
neighboring  islands,  was  covered  deep  with  hot  cin- 
ders, which  completely  destroyed  all  vegetation  and 
left  the  island  a  barren  desert. 

A  visit  was  made  to  the  island  three  years  later, 


The  Problems  of  Plant  Distribution    265 

and  by  that  time  a  good  many  plants  had  already 
established  themselves.  Apparently  the  earliest 
plants  to  get  a  foothold  were  blue-green  algae,  which 
were  found  growing  on  the  barren  cinders,  forming 
black  gelatinous  films  in  which  fern  spores  were  able 
to  germinate,  so  that  nearly  a  dozen  species  of  ferns 
were  noted  on  this  first  visit. 

With  the  rapid  decay  of  the  dead  vegetation,  and 
the  decomposition  of  the  ashes  under  the  sun  and 
rain  of  an  equatorial  climate,  soil  enough  was  soon 
created  for  the  maintenance  of  many  flowering 
plants,  whose  seeds,  borne  by  the  wind,  or  by  birds, 
or  carried  by  the  ocean  currents  to  the  shore,  quickly 
spread  over  the  bare  surface  of  the  island. 

I  had  an  opportunity  of  visiting  Krakatau  in 
April,  1906,  twenty-three  years  after  the  eruption. 
At  this  time  the  island  was  completely  covered  again 
with  vegetation  comprising  a  large  number  of  species 
of  flowering  plants.  Along  the  shore,  the  character- 
istic strand  flora  was  completely  reestablished. 
Fruiting  cocoanut  palms,  Casuarinas,  screw-pines, 
and  various  other  trees,  some  of  them  fifty  feet  in 
height,  formed  a  belt  of  forest,  with  lower  vegeta- 
tion growing  immediately  along  the  beach.  The  flat 
land  between  the  shore  and  the  remains  of  the  cone 
in  the  center  of  the  island,  was  covered  with  a  dense 
growth  of  tall  grasses,  with  a  sparse  growth  of 
shrubs  and  other  plants  between.  In  the  more  shel- 
tered hollows  about  the  base  of  the  cone,  a  dense 
growth  of  young  forest  trees  had  established  itself, 


266  Plant  Life  and  Evolution 

and  probably  in  time  will  extend  itself  to  meet  the 
forest  belt  near  the  shore. 

Professor  Ernst,  who  accompanied  the  party  in 
1906,  has  since  published  a  full  account  of  the  flora 
as  it  was  noted  at  the  time  of  this  visit.  ("  The 
New  Flora  of  Krakatau,"  by  A.  Ernst :  Cambridge 
University  Press,  1909.) 

THE  AGENTS  IN  PLANT  DISTRIBUTION 

Man  as  an  Agent  in  Plant  Distribution. — The 
agents  in  plant  distribution  are  many.  Wind  and 
water  may  be  the  vehicles  of  transportation,  and 
many  animals,  especially  birds,  are  often  the  agents 
of  rapid  dissemination  of  many  seeds  and  fruits 
which  are  often  provided  with  special  organs  facili- 
tating their  distribution.  With  the  rapid  spread  of 
man  into  the  remoter  parts  of  the  earth,  many  plants 
have  been  carried  with  him,  intentionally  or  other- 
wise, and  these  have  often  very  quickly  made  them- 
selves at  home,  and  sometimes  have  driven  out  their 
native  competitors,  very  much  as  the  white  man  has 
driven  out  the  less  fit  savage.  The  rapid  spread  of 
these  imported  plants  often  gives  quite  a  different 
aspect  to  the  region  which  they  have  invaded  from 
what  it  had  before,  and  may  disguise  the  essential 
differences  which  existed  between  it  and  other  re- 
gions. It  is  often  very  hard  to  trace  the  origin  of 
some  of  these  imported  plants,  and  the  task  of  the 
student  of  plant  geography  is  greatly  increased  by 


The  Problems  of  Plant  Distribution    267 

the  presence  of  these  imported  plants  in  nearly  every 
part  of  the  world.  Few  persons  would  imagine  that 
the  daisies,  dandelions,  and  buttercups,  which  span- 
gle the  meadows  and  lawns  of  our  Eastern  States 
are  probably  all  of  them  European  immigrants,  and 
that  the  thistles  and  burdocks  along  the  roadsides 
are  likewise  aliens.  Railways  and  ships  spread  the 
seeds  everywhere,  and  when  the  conditions  are  fa- 
vorable, the  newcomers  quickly  adjust  themselves  to 
their  new  home. 

Barriers  to  Plant  Distribution. — The  great  nat- 
ural barriers  to  plant  migration  are  deserts  and  high 
mountains,  and  large  bodies  of  water,  like  the 
oceans.  The  presence  of  over  a  thousand  miles  of 
desert  and  mountains  between  the  Mississippi  and 
the  Pacific  Coast  largely  explains  the  striking  dif- 
ferences in  the  vegetation  of  California  and  that  of 
the  Atlantic  States. 

While  mountains  are  the  barriers  which  prevent 
the  passage  of  many  plants,  they  may  also  be  high- 
ways along  which  plants  travel.  Thus  the  great 
ranges  of  mountains  running  north  and  south,  per- 
mit the  southward  migration  of  northern  plants,  and 
the  northern  migration  of  antarctic  ones.  Ascend- 
ing higher  and  higher  as  they  go  southward,  many 
arctic  or  north  temperate  plants  have  established 
themselves  on  the  mountains  far  southward  of  their 
original  habitat. 


268  Plant  Life  and  Evolution 

PLANT  DISTRIBUTION  IN  THE  UNITED  STATES 

The  North  American  continent  illustrates  very 
clearly  the  most  important  factors  governing  plant 
distribution,  and  the  United  States  with  its  3,000,000 
miles  of  territory,  reaching  from  the  Atlantic  to  the 
Pacific,  offers  unusual  opportunities  for  studying 
the  most  important  factors  of  Phytogeography. 

The  Eastern  Forest. — In  a  general  way  the 
United  States  may  be  divided  into  three  great  re- 
gions, extending  east  and  west.  East  of  the  Mis- 
sissippi the  country  was  originally  covered  by  an 
almost  unbroken  forest,  of  which  a  very  large  part 
has  disappeared  with  the  clearing  of  the  land,  but 
enough  of  which  remains  even  in  the  more  densely 
settled  regions,  to  make  it  evident  what  was  its 
character;  and  in  the  remoter  districts,  especially 
in  the  southern  mountains,  there  may  still  be  found 
tracts  of  practically  virgin  forest.  This  forest  is 
characterized  by  the  predominance  of  deciduous 
trees,  only  occasionally,  as  in  the  pine-barrens  and 
cypress-swamps  of  the  South,  or  in  the  more  north- 
ern forests,  are  conifers  the  predominant  trees. 

Owing  to  the  absence  of  high  mountains,  or  other 
barriers,  the  flora  of  the  eastern  third  of  the  United 
States  is  remarkably  uniform,  many  species  occupy- 
ing the  whole  area,  the  differences  in  the  different 
portions  being  mainly  those  caused  by  variations  in 
soil,  heat,  or  moisture.  In  the  northern  portion  the 
prevailing  trees  are  oaks,  elms,  maples,  hickories, 


The  Problems  of  Plant  Distribution    269 

etc.,  with  a  mixture  in  places  of  pines,  firs,  hem- 
locks, and  some  other  conifers.  Its  general  char- 
acter approximates  that  of  northern  Europe,  and  it 
merges  into  the  still  more  uniform  forest  flora  of 
the  sub-polar  zone  to  the  North.  But  even  in  the 
more  northerly  territory  of  the  Eastern  United 
States,  types  occur  which  are  quite  absent  from  the 
European  flora.  Hickories,  walnuts,  and  sassafras 
are  extra-European  genera  which  exist  in  Canada, 
and  somewhat  further  south  other  peculiar  forms, 
magnolias,  tulip-trees,  persimmons,  gums,  locusts, 
and  other  less  familiar  types,  absent  from  the  Eu- 
ropean forests,  add  variety  to  the  magnificent  forest 
which  reaches  its  finest  development  on  the  slopes 
of  the  Southern  Appalachian  mountains.  In  the 
southernmost  parts,  e.g.,  Southern  Florida,  a  strong 
tropical  element  derived  from  the  neighboring  West 
Indian  flora  is  conspicuous.  This  includes  such 
forms  as  the  palms,  mahogany,  and  wild  pineapples. 
Some  tropical  types  have  even  made  their  way  far 
north.  Thus  the  pawpaw,  a  member  of  the  tropical 
family  of  Custard-apples,  occurs  as  far  north  as 
Southern  Michigan.  Besides  the  trees,  there  are 
many  beautiful  shrubs  and  herbs  that  characterize 
these  splendid  forests.  Before  the  leaves  appear 
in  the  spring,  many  delicate  herbaceous  plants, 
blood-root,  Dicentra,  spring  beauties,  dog-tooth  vio- 
lets, anemones,  trilliums,  etc.,  rapidly  spring  up,  ex- 
pand their  flowers,  and  as  quickly  disappear,  to 
rest  until  the  next  spring.  Many  flowering  shrubs 


270  Plant  Life  and  Evolution 

adorn  the  woods,  azalea,  and  rhododendron,  syringa, 
honeysuckle,  crab-apples  and  hawthorn,  dogwood 
and  redbud;  and  especially  in  the  southern  woods, 
beautiful  creepers,  grape-vines,  clematis,  wistaria, 
bitter-sweet,  yellow  jasmines,  trumpet-creepers,  and 
passion-flowers,  and  others,  remind  one  of  the  lianas 
of  the  tropics.  Only  in  the  extreme  South  do  we 
encounter  palms,  perhaps  the  most  striking  tree  types 
of  the  tropics. 

Passing  inland  from  the  Atlantic  coast  there  is 
a  marked  diminution  of  the  rainfall,  accompanied 
by  a  corresponding  falling  off  in  the  forest  flora. 
In  Western  Michigan,  Indiana,  and  Illinois  the  for- 
est assumes  a  more  open  character,  and  shows  much 
less  variety  in  the  trees.  Oaks  predominate,  and 
these  "  oak  openings  "  are  very  characteristic  of  the 
territory  abutting  on  the  prairie  region  lying  to 
the  west,  and  small  prairies  already  appear  in  spots 
between  the  forested  areas.  These  patches  of  for- 
est finally  disappear  entirely,  and  the  great  plains 
extending  from  the  Mississippi  to  the  Rocky 
Mountains  are  quite  treeless,  except  along  the 
streams,  where  a  belt  of  cottonwoods  or  willows 
often  marks  the  course  of  some  shallow  muddy 
river. 

The  Great  Plains. — The  great  plains  constitute 
the  second  phytogeographical  region.  A  level  or 
slightly  rolling  plain,  with  meager  rainfall  and  great 
extremes  of  heat  and  cold,  and  with  fierce  winds 
sweeping  it,  the  conditions  are  not  favorable  for 


The  Problems  of  Plant  Distribution    271 

plant  growth,  and  only  particularly  hardy  plants 
can  survive.  First  in  importance  are  the  grasses, 
which  cover  the  entire  eastern  portion  with  a  close 
turf,  but  among  these  there  also  grow  many  beau- 
tiful flowers  which  in  spring  and  summer  dot  the 
prairie  with  spots  of  vivid  color.  Only  along  the 
water  courses,  or  in  sheltered  gullies,  can  trees  find 
a  foothold.  The  soil  of  these  eastern  prairies  is  ex- 
tremely fertile,  and  now  they  are  the  granary  of 
the  whole  country,  immense  fields  of  corn  and  wheat 
covering  the  plains  which  a  generation  ago  were 
unbroken  prairie  sod.  With  the  rapidly  diminishing 
rainfall  westward,  the  close  turf  of  the  eastern  prai- 
ries gives  place  to  arid  expanses,  dotted  with 
bunch-grasses  mingled  with  low  cacti,  sage-brush, 
and  other  outposts  of  the  true  desert  lying  still 
further  west.  These  arid  plains  which  have  risen 
very  gradually,  end  abruptly  in  many  places,  the 
Rocky  Mountains  rising  steeply  from  the  plain 
and  forming  the  beginning  of  the  great  complex  of 
mountain  and  desert  which  reaches  from  the  great 
plains  to  the  Pacific. 

The  Western  Flora. — This  western  third  of  the 
continent  is  in  many  ways  the  most  interesting  of 
all  to  the  botanist,  as  it  presents  a  far  greater  va- 
riety of  conditions  than  prevails  in  the  eastern  half 
of  the  continent.  For  the  most  part  it  is  a  region 
of  light  rainfall,  and  much  of  it  is  a  true  desert 
where  such  plants  as  can  survive  are  extremely 
modified.  Some  mountain  valleys  are  well  watered, 


272  Plant  Life  and  Evolution 

however,  and  the  beautiful  glacial  parks  of  the 
Rocky  Mountains,  adorned  with  luxuriant  mead- 
ows and  fine  forests,  present  a  great  contrast  to  the 
barren  deserts  of  Arizona  and  Nevada.  Except  in 
a  few  places  these  deserts  support  a  scanty,  but  ex- 
tremely characteristic  flora,  which  has  adapted  itself 
to  the  rather  strenuous  conditions  of  desert  life. 
Especially  interesting  are  the  desert  forms  of  the 
Southwest.  Here  the  cacti,  Yucca,  century-plants, 
and  many  other  striking  desert  types  are  especially 
well  developed,  and  in  some  of  the  canyons  opening 
into  the  hot  sandy  waste  of  the  Colorado  desert  in 
Southern  California,  are  groves  of  lofty  palms  that 
might  have  been  transported  bodily  from  the 
tropics. 

The  Pacific  Slope. — The  Pacific  coast  forms  al- 
most a  distinct  botanical  region  of  its  own.  Shut 
off  from  the  country  to  the  east  by  the  great  barrier 
of  the  Cascades  and  the  Sierra  Nevada,  it  shows 
many  peculiarities  in  its  flora,  these  being  particu- 
larly marked  in  California,  where  the  isolation  is 
practically  complete.  This  great  mountain  barrier 
exercises  a  profound  influence  on  the  climate  of  the 
Pacific  coast,  which  is  dominated  by  winds  blowing 
from  the  Pacific,  the  temperature  of  which  varies 
but  little,  so  that  it  acts  as  a  great  thermostat. 
Instead  of  the  sudden  changes  characteristic  of 
the  continental  climate  of  most  of  the  United 
States,  the  Pacific  coast  has  a  climate  which  is  re- 
markably uniform.  The  difference  between  the  mean 


The  Problems  of  Plant  Distribution    273 

of  the  hottest  and  coldest  months  in  San  Francisco 
is  only  10°  F.,  while  in  Washington,  with  nearly 
the  same  latitude,  and  with  the  same  mean  annual 
temperature  (55°),  it  is  44°.  The  rainfall  is  very 
heavy  at  the  north,  but  rapidly  diminishes  towards 
the  south,  so  that  San  Diego,  in  Southern  Cali- 
fornia, receives  only  about  one-tenth  as  much  rain 
as  falls  at  some  points  on  the  northern  coast.  The 
rain  falls  mainly  in  the  winter  and  spring,  the  sum- 
mer being  almost  absolutely  rainless.  These  very 
great  differences  in  climate,  compared  to  Atlantic 
North  America  with  its  cold  winters  and  wet 
hot  summers,  result  in  a  remarkable  difference  in 
the  type  of  vegetation.  Moreover,  in  addition  to 
the  great  differences  in  rainfall  in  different  parts, 
there  must  be  considered  the  differences  in  altitude, 
which  in  California  approach  15,000  feet,  involving 
of  course  very  great  variation  in  temperature  and 
exposure.  Consequently  in  proceeding  down  the 
coast  of  California  from  Humboldt  and  Mendocino 
Counties,  to  the  Colorado  desert  of  the  southeast, 
one  passes  from  forests  of  enormous  trees  with  an 
almost  tropical  luxuriance  of  undergrowth,  to 
barren  deserts  where  no  plant  can  live.  The  mild 
climate  induces  a  growth  of  evergreen  plants,  even 
deciduous  trees  and  shrubs  retaining  their  foliage 
for  the  greater  part  of  the  year. 

All  through  the  West  there  is  a  predominance 
of  coniferous  trees,  this  being  especially  the  case 
in  California,  where  most  of  the  forests  are  com- 


274  Plant  Life  and  Evolution 

posed  of  pines,  firs,  redwoods,  cedars,  etc.  The  de- 
ciduous trees,  like  the  oaks  and  maples,  form,  as 
the  rule,  only  the  undergrowth  for  the  much  taller 
conifers,  except  along  the  streams,  and  sometimes 
in  the  mountain  canyons,  where  often  the  growth 
is  mainly  of  deciduous  trees.  Many  of  the  angio- 
spermous  trees,  however,  are  also  evergreens,  like 
the  live-oaks,  tanbark-oaks,  madrono,  and  laurels, 
and  a  very  characteristic  feature  of  the  drier  forma- 
tions in  California  is  the  dense  scrub,  or  "  chap- 
arral," composed  of  a  great  variety  of  shrubs,  most 
of  them  evergreens,  like  the  manzanita  and  toyon, 
but  with  some  deciduous  species  like  the  buckeye 
and  poison-oak. 

The  Flora  of  California.—  California  illustrates 
very  perfectly  how  important  a  part  topography 
plays  in  the  origin  of  a  flora.  The  Sierra 
Nevada  and  Coast  Ranges  form  perfect  high- 
ways for  the  migration  of  northern  plants,  which 
follow  the  mountains  southward,  ascending  as  they 
go  to  the  altitude  best  fitted  for  them.  In  the 
cool  moist  forests  of  the  outer  Coast  Range  the 
northern  types  are  especially  at  home,  and  many 
of  the  common  flowers,  violets,  trilliums,  spring- 
beauties,  dog-tooth-violets,  Solomon's  seal,  etc.,  are 
closely  related  to  species  that  are  common  in  the 
Eastern  and  Northern  United  States.  With  these 
are  a  few  forms  like  the  fritillaries  and  western 
skunk-cabbage,  and  the  Sitka-spruce,  which  seem 
to  be  immigrants  from  the  Old  World,  via  Alaska. 


The  Problems  of  Plant  Distribution    275 

The  majority  of  the  Calif ornian  plants,  however, 
the  flora  of  the  drier  mountains  and  valleys,  is  of 
Mexican  affinity,  and  the  many  showy  flowers  like 
the  poppies,  cream  cups,  lupins,  nemophila,  Gilia, 
Orthocarpus,  and  Mariposa-lilies,  and  many  others 
which  make  so  splendid  a  showing  in  the  open  val- 
leys and  upon  the  hillsides  of  California  in  the 
spring,  are  for  the  most  part  very  different  from 
any  Eastern  flowers  and  belong  to  the  flora  of  the 
great  Mexican  plateau,  of  which  Arizona  and 
Southern  California  are  really  a  part.  In  Central 
California  the  two  floral  regions  meet,  the  north- 
ern types  of  the  Coast  Range  often  following 
the  stream-beds  down  into  the  valleys,  and  be- 
tween the  hills,  where  the  southern  flora  predomi- 
nates. 

As  in  other  settled  countries,  the  character  of  the 
flora  has  been  modified  by  man  to  a  great  extent. 
California,  however,  being  largely  an  open  country, 
has  not  had  its  flora  so  much  altered  as  was  the 
case  in  the  forested  Eastern  States.  It  is  true  that 
the  forests  have  been  to  some  extent  cut,  but  more 
for  timber  than  for  clearing  the  land,  the  cultivated 
areas  being  mainly  open  country.  Cultivation,  how- 
ever, has  resulted  in  the  introduction  of  very  many 
foreign  plants,  and  by  irrigation  the  desert  has  been 
transformed  into  rich  fields  of  alfalfa,  or  into 
orange  groves  and  vineyards.  With  the  vines  and 
olives  of  the  early  settlers,  came  in  also  many  weeds, 
bur-clover,  wild-oats,  and  mustard,  which  found 


276  Plant  Life  and  Evolution 

themselves  very  much  at  home  in  the  fields  of  Cali- 
fornia, where  they  contested  the  ground  with  the 
native  poppies,  buttercups,  and  lupins,  which,  how- 
ever, manage  to  hold  their  own  pretty  well  against 
the  invaders. 


CHAPTER  IX 

THE  HUMAN  FACTOR  IN  PLANT  EVO- 
LUTION 

THE  changes  in  the  vegetation  of  the  earth  due 
to  the  direct  or  indirect  agency  of  man  have 
been  great  and  far-reaching.  Like  all  other  animals, 
the  very  existence  of  man  is  dependent  upon  plants 
of  some  kind,  and  most  races  of  mankind  rely  upon 
some  single  species  of  plant  as  the  staff  of  life. 
While  the  Eskimos  of  the  frozen  North,  and  the 
nomad  tribes  of  Indians  of  the  western  plains,  de- 
pend mainly  upon  the  products  of  hunting  and  fish- 
ing for  their  existence,  by  far  the  larger  part  of 
mankind  are  vegetarians  in  their  diet,  animal  food 
playing  a  quite  secondary  role.  If  the  original  home 
of  man  was  in  the  eastern  tropics,  as  there  seems 
every  reason  to  believe  was  the  case,  his  natural 
food  would  probably  have  been  mainly  fruits,  seeds, 
and  roots.  Such  shelter  as  was  necessary  for  him 
was  furnished  by  bunches  of  branches  or  leaves,  or 
mud  huts  thatched  with  grass  or  palm-leaves,  such 
as  we  now  find  among  many  of  the  lower  savage 
tribes.  Even  at  the  present  day,  wild  plants  yield 
a  not  unimportant  source  of  food  for  mankind. 
277 


278  Plant  Life  and  Evolution 

Wild  fruits,  like  the  nuts  and  berries  of  temperate 
climes,  are  by  no  means  despised  even  by  the  most 
civilized  peoples,  and  the  savages  of  the  more  prolific 
tropical  zones  depend  very  largely  for  their  sub- 
sistence upon  the  fruits  and  roots  growing  spon- 
taneously in  the  forest.  Man  also,  at  a  very  low 
stage  in  his  development,  learned  to  use  the  tenacious 
fibers  of  many  wild  plants  for  clothing  and  for 
other  purposes.  In  its  most  primitive  form  this  still 
survives  in  the  "  tapa  "  or  bark  cloth  of  the  South 
Sea  Islanders.  Spinning  and  weaving  \vere  much 
later  achievements.  A  few  wild  fiber  plants  are  still 
of  some  commercial  value,  the  most  important  of 
these  probably  being  the  wild  flax  (Phormium 
tenax}  of  New  Zealand,  which  is  manufactured  in 
considerable  quantities,  and  forms  an  important  arti- 
cle of  export. 

As  primitive  man  migrated  from  the  fertile  trop- 
ical forests,  where  he  had  his  birth,  to  regions  less 
prolific  in  wild  food  plants,  he  was  probably  driven 
to  feeding  on  a  much  greater  variety  of  food  than 
was  his  early  habit,  and  we  may  assume  that  his 
more  marked  carnivorous  tastes  were  gradually  de- 
veloped. At  the  present  day  there  are  savage  tribes 
whose  food  supply  must  be  quite  as  precarious  as 
that  of  their  ancient  forebears.  The  inhabitants  of 
the  far  North  and  the  South,  or  such  degraded  sav- 
ages as  the  native  Australians,  eat  pretty  nearly 
anything  which  they  can  procure,  and  in  times  of 
scarcity  are  often  driven  to  feed  upon  most  un- 


Human  Factor  in  Plant  Evolution     279 

wholesome  plants.  Some  of  the  native  Californian 
Indians  still  collect  acorns,  pine-nuts,  and  such 
other  wild  seeds  and  berries  as  they  can  gather, 
with  grasshoppers,  caterpillars,  and  other  similar 
small  game  for  variety. 

Such  wild  fruits  as  the  strawberry,  huckleberry, 
cranberry,  persimmon,  nuts,  etc.,  are  much  esteemed 
by  everybody,  but  can  hardly  be  considered  as  im- 
portant articles  of  diet.  Among  the  savage  tribes, 
however,  these  wild  fruits  and  seeds  may  be  the 
staple  sources  of  their  food  supply.  The  Indians 
of  the  Great  Lake  region  used  regularly  to  harvest 
the  wild  rice,  and  the  Californian  Indians  looked 
upon  the  oaks  and  nut-pines  as  their  harvest  fields. 
The  South  Sea  Islander  gathers  cocoanuts  or  bread- 
fruit, and  most  savages  depend  to  a  greater  or  less 
extent  upon  the  spontaneous  products  of  a  more  or 
less  generous  nature. 

In  the  tropics  especially,  it  is  hard  to  draw  the 
line  between  cultivated  and  wild  plants,  as  so  many 
of  the  cultivated  fruits,  like  oranges,  mangoes,  and 
bananas,  readily  escape  from  cultivation  and  are 
often  found  growing  far  from  any  cultivated 
ground,  offering  their  fruits  to  whoever  may  care 
to  gather  them. 

Origin  of  Agriculture. — The  development  of 
agriculture  must  have  been  very  gradual  and  largely 
a  matter  of  chance.  It  is  more  than  probable  that 
even  in  the  earliest  stages  of  agriculture  and  horti- 
culture, there  appeared  quite  accidentally  varieties  of 


280  Plant  Life  and  Evolution 

fruits  or  grains  which  were  sufficiently  superior  to 
the  common  wild  forms  to  attract  the  attention  of 
the  primitive  husbandman,  who  would  naturally  pre- 
fer these,  and  at  the  same  time  may  have  taken 
the  trouble  to  plant  the  seeds  of  these  superior 
forms,  thus  inaugurating  a  most  important  epoch 
in  the  history  of  mankind,  since  the  development  of 
agriculture  made  it  possible  for  man  to  spread  to 
regions  which  without  cultivation  would  have  been 
unable  to  support  him. 

Succeeding  the  more  or  less  casual  planting  about 
his  dwelling  of  wild  fruit-trees,  it  may  be  surmised 
that  primitive  man  began  to  follow  methods  of  agri- 
culture approximating  some  of  those  in  vogue 
among  savage  races  at  the  present  day,  and  indeed 
not  entirely  unknown  to  the  white  man.  The  native 
of  the  tropics — and  of  the  Southern  Alleghanies — 
still  girdles  the  trees  so  as  to  make  a  clearing  in  the 
forest,  and  plants  his  crops  in  the  space  thus  opened 
to  the  sunlight.  After  a  few  crops  have  been  taken 
off  from  this  clearing,  it  is  deserted  and  another 
one  made.  Nature  promptly  repairs  the  damage, 
and  another  generation  sees  the  forest  again  in  pos- 
session. 

The  extension  of  agriculture  to  more  arid  re- 
gions necessitated  more  careful  methods,  and  it  was 
especially  in  such  regions  that  the  most  scientific 
methods  of  agriculture,  including  irrigation,  had 
their  birth,  and  thus  were  made  possible  the  civiliza- 
tions of  Babylonia  and  Egypt,  Mexico  and  Peru. 


Human  Factor  in  Plant  Evolution     281 

Antiquity  of  Certain  Cultivated  Plants. — As  far 
back  as  history  runs  we  encounter  evidences  of  the 
cultivation  of  many  food  and  textile  plants  still  in 
use.  This  is  true  in  Egypt,  China,  and  India,  and 
in  the  New  World  the  advanced  civilization  shown 
by  remains  of  man  in  Peru  and  Mexico,  show  that 
agriculture  had  reached  a  high  degree  of  perfection 
at  a  very  remote  period.  Among  the  most  inter- 
esting evidences  as  to  the  early  cultivation  of  many 
useful  plants  in  prehistoric  times,  are  the  remains 
of  the  Swiss  Lake-dwellers.  At  a  period  when 
these  ancient  people  still  used  only  stone  implements, 
they  cultivated  a  variety  of  food  plants,  including 
wheat,  barley,  and  millet,  and  they  also  grew  flax. 

First  in  importance  among  cultivated  plants  are 
those  grown  for  food,  and  among  these  the  various 
grains  take  first  place,  followed  by  certain  fruits; 
but  other  parts  of  the  plants — the  leaves,  roots,  or 
tubers — may  also  be  important  sources  of  food. 
Next  in  importance  are  plants  furnishing  fibers,  usu- 
ally derived  from  stems  or  leaves,  the  most  im- 
portant exception  being  cotton,  which  furnishes  a 
fiber  attached  to  the  seed.  Some  of  these  fiber 
plants,  especially  flax  and  hemp,  have  been  cultivated 
from  the  earliest  historical  times,  but  the  history 
of  some  of  the  less  familiar  fibers,  like  jute,  Manila 
hemp,  and  ramie,  is  very  imperfectly  known. 

It  is  probable  that  very  early  in  his  history,  man 
learned  the  virtues  of  certain  medicinal  plants.  In 
his  search  for  food  plants  he  doubtless,  through 


282  Plant  Life  and  Evolution 

more  or  less  unpleasant  experience,  discovered  the 
narcotic  properties  of  opium  and  tobacco,  the  stimu- 
lating effects  of  tea  and  coffee,  the  cathartic  quali- 
ties of  various  seeds  and  barks.  While  medicinal 
plants  must  be  considered  of  much  less  importance 
than  the  staple  food  plants,  still  they  are  in  many 
cases  of  very  great  value,  as  for  instance  quinine, 
which  has  made  it  possible  for  white  men  to  live 
in  certain  fever-stricken  regions  in  the  tropics  where 
otherwise  they  could  not  have  existed. 

The  last  category  of  useful  plants  comprises  those 
grown  for  constructive  purposes,  like  timber  trees 
and  bamboo,  but  even  at  the  present  day  the  supply 
of  timber  is  largely  drawn  from  natural  forests — 
tree  planting  for  timber  being  a  recent  development, 
but  one  of  rapidly  growing  importance  and  likely 
to  affect  strongly  the  character  of  large  tracts  of 
country  in  the  United  States  and  elsewhere.  .  .  . 

Just  when  the  first  attempts  at  agriculture  were 
made  is  not  known,  but  it  is  evident  from  historical 
record  that  in  all  the  ancient  civilizations  of  which 
we  have  any  trace,  many  plants  were  regularly  culti- 
vated. Thus  in  ancient  Egypt  wheat  and  flax  were 
grown,  and  in  China,  5,000  years  ago,  there  is  evi- 
dence that  rice,  wheat,  sweet  potatoes,  and  millet 
were  cultivated.  In  America,  long  before  its  dis- 
covery by  Europeans,  Indian  corn,  potatoes,  and 
tobacco,  and  other  plants  were  regularly  grown, 
but  very  little  is  known  of  the  exact  age  of  these 
early  civilizations  in  South  America  and  Mexico. 


Human  Factor  in  Plant  Evolution   283 

All  but  the  lowest  savages  and  certain  nomad  races 
cultivate  some  forms  of  plants  for  food,  but  in  many 
cases  these  have  been  cultivated  from  the  earliest 
historic  times  and  have  become  so  altered  by  culti- 
vation that  their  origin  is  a  matter  of  very  great 
uncertainty. 

It  is  in  the  improvement  and  subsequent  changes 
of  the  food  plants  that  man's  influence  as  a  creator 
of  new  plant  forms  is  most  clearly  seen.  The  de- 
velopment of  new  ornamental  plants,  except  in  very 
recent  years,  has  been  relatively  unimportant,  and 
practically  all  of  these  are  clearly  traceable  to  wild 
ancestors  which  still  exist.  Many  wild  fruits  like 
strawberries,  raspberries,  crab-apples,  etc.,  have  been 
brought  under  cultivation  but  have  been  altered 
comparatively  little.  In  some  of  these  cultivated 
forms  it  is  evident  that  two  or  more  species  have 
been  crossed,  and  this  sometimes  makes  it  difficult 
to  be  quite  certain  as  to  their  origin.  Many  tropical 
fruits,  such  as  the  mango,  mangosteen,  cocoanut, 
durian,  custard-apple,  and  many  others,  are  good 
botanical  species,  existing  but  little  changed  in  the 
wild  state.  None  of  these,  however,  can  be  looked 
upon  as  the  main  source  of  food  supply,  and  perhaps 
this  accounts  for  the  comparatively  slight  changes 
that  they  have  undergone  in  cultivation.  The  vari- 
ous fiber  plants,  like  flax,  hemp,  and  cotton,  have 
been  but  little  altered,  since  the  flowers  and  fruit  do 
not  influence  in  any  way  the  character  of  the  fiber. 
It  is  evident  that  prehistoric  man  utilized  the  fiber 


284  Plant  Life  and  Evolution 

of  many  plants,  first  probably  for  fish-lines  and  for 
binding  the  heads  of  arrows  and  spears  and  axe- 
heads  to  the  shafts,  etc.,  but  he  later  learned  the  art 
of  spinning  and  weaving,  to  provide  himself  with 
clothing.  The  ancient  Egyptians  knew  the  art  of 
weaving,  and,  as  we  have  seen,  there  is  evidence 
that  even  the  Swiss  Lake-dwellers  cultivated  a  spe- 
cies of  flax. 

Forage  Plants. — Besides  the  plants  used  directly 
by  man  for  food,  there  must  be  taken  into  account 
the  forage  plants,  which  serve  to  nourish  the  animals 
upon  which  man  feeds.  These,  like  the  staple  food 
plants  of  mankind,  are  largely  grasses,  most  of  the 
herbiverous  animals,  like  cattle,  sheep,  etc.,  feeding 
largely  upon  grasses  of  various  kinds.  Next  in  im- 
portance as  forage  plants  are  various  leguminous 
plants,  clover,  alfalfa,  etc.  These  have  become  little 
altered  by  cultivation  and  to  all  intents  and  pur- 
poses are  still  wild  plants. 

Cereals. — The  food  staple  of  nearly  all  peoples 
is  some  form  of  farinaceous  vegetable  food,  and 
this  is  seen  in  its  greatest  perfection  in  the  grains 
yielded  by  various  species  of  grasses,  which  from 
very  early  times  have  formed  the  principal  food  for 
the  vast  bulk  of  mankind.  Some  of  these  grains 
have  been  in  cultivation  as  far  back  as  there  is  any 
historical  record,  and  indeed,  as  we  have  seen,  there 
are  evidences  of  the  cultivation  of  grains  even  in 
prehistoric  times,  as  shown  by  remains  of  the  Swiss 
Lake-dwellers  of  the  stone  age.  In  consequence  of 


Human  Factor  in  Plant  Evolution   285 

the  enormously  long  period  during  which  these  have 
been  under  cultivation,  they  have  lost  most  of  their 
original  characters,  and  it  is  almost  impossible  to 
determine  what  their  progenitors  were,  and  whether 
they  still  exist  in  the  wild  state.  It  has  been  argued 
from  the  rapidity  with  which  plants  are  known 
to  change  under  cultivation,  that  the  ancestors  of 
some  of  the  cultivated  grains  may  still  exist  in  the 
wild  state,  but  so  different  from  their  cultivated  de- 
scendants as  not  to  be  recognizable.  The  most 
important  of  these  grains  are  wheat,  rice,  and 
maize. 

There  is  much  doubt  as  to  the  origin  of  the  dif- 
ferent cultivated  forms  of  wheat,  and  it  is  still  a 
question  whether  they  really  all  belong  to  a  single 
species.  Probably  they  were  derived  from  some 
species  inhabiting  the  region  of  the  Euphrates,  and 
perhaps  also  parts  of  Southeastern  Europe,  but  just 
exactly  what  these  species  were,  is  by  no  means 
clear.  Rice  has  been  cultivated  from  the  earliest 
historic  times  in  India  and  China,  and  wild  rice  ap- 
parently specifically  identical  with  the  cultivated 
plant  still  grows  in  India.  It  might  be  said  here 
that  the  "  wild  rice  "  of  the  Eastern  United  States, 
which  was  so  important  an  article  of  food  among 
certain  of  the  North  American  Indians,  is  an  en- 
tirely different  plant  from  the  Oriental  rice.  Maize, 
the  staple  grain  of  the  New  World,  has  been  culti- 
vated from  the  earliest  times  of  which  there  are  any 
records,  both  in  Mexico  and  South  America.  It  is 


286  Plant  Life  and  Evolution 

unknown  in  the  wild  condition,  but  there  are  certain 
grasses  related  to  maize  which  it  has  been  surmised 
may  possibly  be  the  wild  plants  from  which  the  culti- 
vated maize  has  arisen. 

Fruits. — In  some  countries  grains  are  replaced 
to  a  great  extent  by  fruits  like  the  banana 
and  breadfruit,  or  tubers  like  the  taro  or  the  po- 
tato. The  banana  in  its  many  forms  has  been 
cultivated  from  the  earliest  times  in  the  Asiatic 
tropics,  and  Humboldt  states  that  it  was  culti- 
vated in  America  prior  to  its  discovery  by 
Columbus;  but  this  seems  to  be  very  doubtful, 
as  all  wild  bananas  are  Asiatic  and  it  seems 
practically  certain  that  the  banana  was  introduced 
into  America  shortly  after  its  discovery  by  Euro- 
peans. As  the  cultivated  bananas  are  seedless,  their 
spread  into  foreign  countries  is  absolutely  dependent 
upon  human  agency.  The  same  is  true  of  the 
breadfruit,  which  has  been  carried  from  its  home 
in  Java  to  all  of  the  moister  regions  of  the  trop- 
ics. It  is  quite  common  to  find  the  breadfruit  and 
banana  growing  far  away  from  human  habitations, 
and  this  is  true  also  of  the  taro,  which  is  known  in 
the  wild  state  in  the  warmer  parts  of  the  Indo- 
Malayan  regions.  It  has  been  carried  by  the  Poly- 
nesians to  all  the  warmer  parts  of  the  South  Seas, 
and  is  still  a  very  important  article  of  diet  among 
the  Hawaiians.  The  potato  was  cultivated  in  Amer- 
ica, especially  in  the  mountain  regions  of  South 
America,  long  before  the  advent  of  the  Europeans. 


Human  Factor  in  Plant  Evolution     287 

It  still  exists  in  the  wild  state  in  Chili  and  possibly 
in  the  mountainous  regions  to  the  north. 

Man's  Spread  over  the  Earth  Due  to  Agriculture. 
— With  the  adoption  of  agricultural  habits,  the  pos- 
sibilities of  man's  expansion  over  very  wide  areas 
of  the  earth's  surface  became  possible,  and  as  a  re- 
sult of  his  migrations  the  vegetation  of  the  invaded 
regions  has  been  very  greatly  altered.  The  clearing 
of  large  tracts  for  agricultural  purposes,  and  the 
replacement  of  native  plants  by  cultivated  ones,  has 
very  much  changed  the  aspect  of  great  areas  of  the 
earth's  surface  all  over  the  world.  Little  of  Europe 
is  in  a  state  of  nature,  and  the  same  is  true  of  south- 
ern and  eastern  Asia  and  northern  Africa.  In  the 
old  settled  countries,  like  most  of  Europe  and  much 
of  Asia,  all  traces  of  the  original  forest  have  long 
ago  disappeared,  and  one  must  go  to  the  most  re- 
mote mountain  regions  to  find  any  remains  of  it. 

In  the  tropical  regions  the  jungle  quickly  grows  up 
again  when  cultivation  is  neglected,  but  this  is  not 
the  case  in  the  colder  and  drier  climates  of  most 
parts  of  Europe.  In  the  United  States,  the  greater 
part  of  the  dense  forest  of  the  Atlantic  region  has 
given  way  to  great  cities,  or  to  fields,  meadows,  and 
orchards  largely  occupied  by  alien  plants — wheat, 
corn,  clover,  and  fruit  trees,  of  various  kinds,  and  all 
the  familiar  garden  flowers  and  vegetables.  These 
are,  with  very  few  exceptions,  foreigners,  which 
have  replaced  the  native  forest  trees  and  under- 

frnwtli         TVTnrpnvpr      with    thpsp    intrnrlnrprl      rulti- 


288  Plant  Life  and  Evolution 

vated  plants  have  crowded  in  hordes  of  less  welcome 
immigrants,  the  troops  of  foreign  weeds  which  have 
taken  possession  of  all  the  waste  places  along  the 
roadsides,  fence  corners,  and  other  places  not 
monopolized  by  the  crops.  Most  of  the  more  ag- 
gressive weeds,  thistles,  dandelions,  burdocks,  sor- 
rel, etc.,  are  of  European  origin,  these  hardy  in- 
vaders ousting  the  delicate  shade-loving  native 
plants  which  thrive  only  in  the  shelter  of  the  dense 
forest.  Only  in  the  swamps  and  other  similar  open 
places  do  the  native  plants  hold  their  own  against 
the  foreign  invaders.  .  .  . 

Introduced  Plants. — It  not  infrequently  happens 
that  plants  escape  from  cultivation  and  find  them- 
selves so  much  at  home  that  they  have  all  the  ap- 
pearances of  natives,  and  this  has  led  to  many  er- 
rors in  determining  the  origin  of  many  cultivated 
species.  Thus  the  orange  grows  spontaneously  in 
the  forests  in  Florida  and  Jamaica,  and  is  to  all 
appearances  wild,  but  we  know  that  it  is  a  native 
of  the  Old  World  and  was  unknown  in  America  be- 
fore the  advent  of  the  Spaniards.  The  banana 
also  is  often  met  with  in  the  forests  of  nearly  all 
tropical  countries,  but  there  is  no  question  that  it  is 
an  escape  from  cultivation,  and  the  same  is  true 
undoubtedly  of  many  other  tropical  fruits,  like  the 
mango  and  guava.  The  readiness  with  which  culti- 
vated plants  adapt  themselves  to  their  new  homes, 
makes  the  discovery  of  their  real  origin  often  a 
matter  of  very  great  difficulty. 


Human  Factor  in  Plant  Evolution     289 

In  the  case  of  primitive  man,  especially  in  the 
tropics,  the  effects  of  cultivation  were  probably  in- 
significant. A  few  acres  of  forest  burned  off,  or 
the  trees  girdled  and  left  to  die,  furnished  the  crude 
field  where  he  planted  his  crop  of  yams  and  corn, 
and  after  a  few  crops  had  been  taken  off,  the  plot 
was  left  to  revert  to  forest.  Very  different  has 
been  the  case  in  modern  times,  where  man  has 
spread  over  the  whole  world,  and  profoundly 
changed  the  character  of  the  vegetation  of  vast 
areas  of  the  earth's  surface.  These  changes  have 
been  going  on  steadily  in  many  parts  of  the  Old 
World  for  many  centuries,  but  it  is  in  the  more  re- 
cently settled  regions,  like  the  United  States  and 
Australia,  that  the  great  changes  in  the  vegetation 
due  to  the  invasion  of  man  can  be  best  appreciated. 
Perhaps  the  United  States,  more  than  any  other 
country,  will  illustrate  this  most  vividly,  owing  to 
the  great  rapidity  with  which  it  has  been  settled 
during  the  past  century. 

Changes  in  America  Due  to  Cultivation. — A  hun- 
dred years  ago,  all  but  a  small  part  of  the  Atlantic 
third  of  the  United  States  was  an  almost  unbroken 
forest,  with  very  little  open  land  except  marshes 
and  swamps,  and  in  the  western  parts,  the  small 
prairies,  which  were  the  outposts  of  the  great  plains 
of  the  trans-Mississippi  region.  Aside  from  the 
trees,  the  undergrowth  consisted  of  shrubs  and 
herbs  fitted  for  the  most  part  to  growth  in  the 
dense  shade  of  the  forest,  and  quite  unadapted  to 


290  Plant  Life  and  Evolution 

survive  the  clearing  away  of  the  forest  cover.  Be- 
fore the  ax  of  the  pioneer  these  forests  rapidly  dis- 
appeared, and  the  clearings  were  planted  with  the 
crops  upon  which  he  depended  for  food,  or  were 
allowed  to  run  to  grass  for  the  subsistence  of  his 
herds  and  flocks.  In  these  clearings  the  delicate 
plants  of  the  shady  forest  perished,  and  the  waste 
places  were  gradually  invaded  by  hordes  of  for- 
eign weeds,  brought  in  the  seed,  or  carried  on  the 
coats  of  the  stock  or  in  the  belongings  of  the  immi- 
grant. These  hardy  foreigners  have  now  so  estab- 
lished themselves  that  any  one  but  the  botanist  takes 
for  granted  that  they  are  natives.  Few  people  real- 
ize that  the  majority  of  our  familiar  weeds,  the 
dandelions,  daisies,  buttercups,  etc.,  are  European 
immigrants.  These  in  many  cases  have  proved 
themselves  so  well  fitted  to  their  new  home,  that  they 
have  almost  monopolized  the  waste  places,  and  have 
invaded  the  cultivated  lands,  so  that  they  have  be- 
come pestilent  weeds.  It  is  hard  to  realize  that  little 
more  than  300  years  ago  there  were  none  of  these 
common  weeds  to  be  met  with  in  America.  With  the 
rapid  extension  of  the  settlement  of  the  western 
plains,  due  to  the  opening  up  of  railroads,  which 
offered  rapid  transit  for  man  and  also  for  weeds, 
very  new  conditions  were  met  with.  The  exposed 
prairie  was  the  home  of  many  hardy  plants  fitted 
to  live  in  the  open,  and  many  of  these  prairie  weeds 
— sunflowers,  yellow  ox-eye  daisies,  ragweed,  and 
many  others,  migrated  eastward  and  joined  the 


Human  Factor  in  Plant  Evolution     291 

army  of  European  weeds,  with  which  they  managed 
to  compete  pretty  successfully,  and  now  mingle  with 
them  on  an  equal  footing  in  the  floral  display  of 
the  meadows  and  roadsides  of  the  New  England 
States. 

Sometimes  plants  that  have  been  introduced  for 
ornament  or  for  useful  purposes,  prove  so  well 
adapted  to  their  new  home  that  they  escape  from 
cultivation,  and  may  become  a  veritable  pest,  just 
as  in  the  animal  kingdom  the  rabbits  in  Australia, 
and  the  sparrows  in  America,  have  proved  alto- 
gether too  well  fitted  to  their  new  homes.  A  good 
instance  of  this  is  found  in  the  Hawaiian  Islands, 
where  a  rather  pretty  garden  shrub,  Lantana,  has 
spread  over  all  the  drier  lowlands  so  that  it  now 
has  become  a  real  nuisance.  It  is  said  that  the  Mina, 
a  bird  introduced  from  India,  is  largely  responsible 
for  distributing  the  seeds  of  the  Lantana.  In  New 
Zealand,  the  sweet-briar  and  gorse,  introduced  by 
the  early  British  settlers,  have  similarly  escaped 
from  cultivation  and  become  troublesome  weeds,  and 
many  similar  cases  can  be  cited  from  various  parts 
of  the  world. 

Deserted  Land  Returning  to  Forest. — As  the 
arable  lands  of  the  East  have  been  exhausted,  and 
deserted  for  the  rich  prairie  farms  of  the  Middle 
West,  they  have  often  been  abandoned,  and  now 
Nature  is  trying  to  repair  to  some  extent  the  rav- 
ages made  by  man.  Many  of  the  deserted  farms  in 
New  England  and  New  York  have  rapidly  reverted 


292  Plant  Life  and  Evolution 

to  forest,  and  vigorous  growths  of  the  same  trees — 
pines,  oaks,  maples,  and  walnuts — that  once  covered 
the  whole  region  with  an  unbroken  forest,  are  again 
taking  possession  of  the  soil  which  for  a  hundred 
years  or  more  was  devoted  to  agriculture. 

Effects  of  Clearing  the  Forest. — The  clearing  of 
the  country  has  also  affected  the  vegetation  in  an- 
other way.  By  the  removal  of  the  forest  cover, 
especially  in  the  mountainous  regions,  there  has 
often  resulted  a  disastrous  denudation  of  the  soil, 
due  to  the  washing  of  heavy  rains,  and  to  landslides. 
The  washing  away  of  the  fertile  surface  soil  makes 
it  impossible  for  many  plants,  which  formerly  oc- 
cupied these  places,  to  grow,  and  the  bare  slopes  can 
only  support  species  which  are  fitted  to  live  in  an 
impoverished  soil.  The  drying  up  of  the  lowlands, 
due  also  to  the  interference  with  the  water  supply, 
the  result  from  the  clearing  off  of  the  forest,  must 
necessarily  affect  very  strongly  the  vegetation  of  the 
region.  There  is  little  evidence  that  the  total  amount 
of  rain  in  a  forested  region  is  materially  diminished 
by  clearing  the  land;  but  the  effect  upon  the  flow 
of  springs  and  streams  is  very  marked.  On  a  for- 
ested mountain  side  the  shade  of  the  trees  checks 
the  evaporation  from  the  soil,  and  the  undergrowth 
and  spongy  masses  of  decaying  leaves  and  twigs 
allow  the  water  to  percolate  slowly  through,  reach- 
ing the  sources  of  the  springs  and  streams  gradu- 
ally and  keeping  the  flow  steady.  When  the  land 
is  cleared,  the  water  runs  off  quickly,  making  the 


Human  Factor  in  Plant  Evolution     293 

streams  raging  torrents  after  heavy  rainfalls  and 
when  the  snow  melts,  only  to  leave  them  shrunken 
into  insignificance  in  the  heat  of  summer.  This  dis- 
turbance of  the  water  distribution  necessarily  affects 
very  strongly  the  vegetation  of  the  region  con- 
cerned. Of  perhaps  as  much  interest  scientifically 
as  practically,  is  the  result  of  extensively  reclaiming 
swamp  areas.  The  swamps  and  bogs  are  the  haunts 
of  many  of  the  rarest  and  most  beautiful  of  our 
native  plants,  which  have  taken  refuge  in  these  in- 
accessible sanctuaries.  The  tamarack  swamp,  with 
its  beds  of  peat-mosses  and  dense  undergrowth,  was 
the  happy  hunting  ground  of  the  botanist.  Now 
with  the  draining  of  the  bogs,  there  are  rapidly  dis- 
appearing many  of  our  loveliest  orchids,  the  car- 
dinal flower,  pitcher-plants,  and  hosts  of  other  curi- 
ous and  beautiful  botanical  treasures. 

Introduction  of  Foreign  Plants. — The  widespread 
introduction  of  ornamental  trees  and  garden  flowers 
into  civilized  countries  has  also  much  changed  the 
appearance  of  the  vegetation  in  all  of  them.  In  any 
long-settled  community  it  is  astonishing  how  little 
of  the  vegetation  which  one  encounters  is  really 
native  to  it,  or  if  so,  has  not  been  planted  by  man. 
Indeed  at  present,  if  one  wishes  to  see  the  unchanged 
indigenous  vegetation  of  any  country,  it  is  neces- 
sary to  seek  the  most  remote  and  unsettled  regions 
of  swamp,  moor,  or  mountain. 

The  origin  of  many  cultivated  plants,  as  we  have 
seen,  is  very  obscure,  and  it  is  evident  that  most  of 


294  Plant  Life  and  Evolution 

the  common  domestic  plants  have  been  very  much 
changed  in  the  course  of  the  centuries  during  which 
they  have  been  cultivated.  It  is  highly  probable 
that  the  earliest  agriculturalists  took  advantage  of 
the  variations  which  occur  in  wild  plants,  and  that 
this  process,  in  time,  resulted  in  the  widely  different 
varieties  which  have  replaced  the  primitive  stocks, 
which  in  many  cases  we  can  no  longer  with  cer- 
tainty recognize.  Hybridization  has  also  undoubt- 
edly played  an  important  part  in  the  origin  of  many 
cultivated  races  of  plants,  but  it  is  not  likely  that 
this  was  consciously  practised  in  early  times,  al- 
though it  is  not  improbable  that  hybrids  may  have 
been  responsible  for  some  of  the  early  cultivated 
plants. 

Plant  Breeding. — Of  late  years,  however,  the  de- 
velopment of  new  forms  of  plants  has  been  the  de- 
liberate aim  of  a  host  of  experimenters,  and  hun- 
dreds or  even  thousands  of  well-marked  varieties, 
often  much  more  different  in  appearance  from  each 
other  than  are  many  natural  species,  have  resulted 
from  their  labors.  One  has  but  to  consider  the  enor- 
mous number  of  new  varieties  of  almost  any  popular 
flower  or  fruit — apples,  grapes,  roses,  narcissi, 
etc.,  which  the  catalogues  advertise  every  year,  to 
realize  the  part  which  man  has  deliberately  played  as 
a  creator  of  new  forms  of  plants.  These  may  be  the 
result  of  spontaneous  variation  and  subsequent  se- 
lection, or  by  skilful  crossing  of  different  species  or 
varieties,  the  tendency  to  variation  may  be  very 


Human  Factor  in  Plant  Evolution     295 

greatly  increased,  and  a  wider  range  of  variation 
may  be  thus  developed,  of  which  the  experimenter 
takes  advantage.  It  is  in  this  way  that  man  has 
most  conspicuously  acted  as  a  real  creator  of  new 
plant  forms,  many  of  which,  as  we  have  said,  are 
very  different  from  any  natural  species.  How  far 
plants  naturalized  for  a  long  time  in  a  new  country, 
either  intentionally  or  otherwise  through  human 
agency,  have  become  permanently  altered,  has  been 
but  little  investigated ;  but  it  would  certainly  be  very 
interesting  to  know  whether  or  not  weeds,  for  ex- 
ample, after  two  or  three  centuries,  have  diverged 
perceptibly  from  the  type  of  the  same  species  grow- 
ing in  the  original  habitat,  and  whether  such  differ- 
ences would  be  lost  if  these  plants  were  grown  for 
a  series  of  years  in  their  old  home. 

Changes  in  European  Plants  Introduced  into 
America. — It  is  well  known  that  the  standard  fruits 
introduced  into  America  from  Europe  have  varied 
extremely,  without  any  conscious  selection  on  the 
part  of  man.  This  has  been  very  carefully  studied 
with  reference  to  apples,  especially,  many  varieties 
of  which  were  introduced  originally  from  Europe. 
By  1817,  according  to  Professor  Bailey,  over  60 
per  cent  of  the  best  varieties  in  the  United  States 
were  of  American  origin,  and  at  present  probably 
over  80  per  cent  are  native  varieties.  Most  of  these 
American  types  do  not  succeed  in  England,  owing 
to  the  different  climatic  conditions.  It  is  quite 
probable  that  a  similar  study  of  the  accidentally  in- 


296  Plant  Life  and  Evolution 

troduced  plants,  like  the  weeds,  would  show  much 
the  same  differences  as  a  result  of  adaptation  to 
the  new  climatic  conditions,  but  it  is  not  likely 
that  these  differences  would  be  nearly  so  marked  as 
in  the  cultivated  species,  which  are  notoriously 
variable. 

While  from  very  early  times  superior  forms  of 
fruits  and  flowers,  which  may  have  arisen  by 
chance,  were  selected  for  cultivation,  deliberate  at- 
tempts to  produce  new  forms  by  crossing,  or  special 
methods  of  cultivation  and  selection,  seem  to  have 
been  first  practised  towards  the  end  of  the  i8th 
century.  It  is  highly  probable,  however,  that  the 
Japanese  and  Chinese,  who  are  such  skilful  horti- 
culturists, practised  this  art  long  before  scientific 
methods  arose  in  Europe  and  America,  but  reliable 
information  on  this  point  is  difficult  to  secure.  The 
importance  of  selecting  the  best  type  of  seed,  in 
order  to  maintain  an  excellence  in  any  strain,  has 
been  recognized  from  very  early  times,  but  that 
man  is  able  to  actually  create  new  forms  of  life 
was  not  realized  until  quite  recently. 

Early  European  Plant-Breeders. — Among  the  ear- 
liest European  horticulturists  who  worked  along 
really  scientific  lines,  there  were  two  whose  experi- 
ments were  made  during  the  latter  part  of  the  i8th 
century  and  the  early  part  of  the  igth  century. 
These  two  men,  Van  Mons  in  Belgium  and  Knight 
in  England,  developed  systems  of  plant-breeding 
which  resulted  in  the  production  of  many  valuable 


Human  Factor  in  Plant  Evolution     297 

fruits,  and  had  a  great  influence  upon  the  methods 
of  plant-breeding. 

With  the  breaking  down  of  the  dogma  of  the 
fixity  of  species,  especially  due  to  Darwin's  work, 
an  impetus  was  given  to  experiments  in  originating 
new  forms.  Some  of  the  results  of  these  may  be 
noted  in  the  long  list  of  new  varieties  of  fruits, 
vegetables,  and  flowers,  that  appear  in  the  florists' 
catalogues  every  year.  Plant-breeding  has  now  be- 
come a  science,  and  the  results  of  these  experiments 
are  often  of  quite  as  great  importance  scientifically, 
as  they  are  commercially. 

The  history  of  cultivated  plants  in  the  United 
States  is  an  interesting  one.  Most  of  the  standard 
fruits,  vegetables,  and  field  crops  were  first  brought 
from  Europe  to  the  Atlantic  States;  but  in  course 
of  time  the  marked  climatic  differences  between  the 
United  States  and  Western  Europe  began  to  change 
the  characters  of  most  of  the  cultivated  plants,  and 
new  varieties  appeared  which  departed  so  markedly 
from  the  parent  stock  that  they  were  soon  given 
special  names.  By  the  selection  of  the  best  of  these 
native  seedlings,  most  of  the  plant  varieties  now  in 
cultivation  have  arisen.  At  present,  at  least  in  the 
Eastern  States,  comparatively  few  European  varie- 
ties are  grown,  this  being  especially  the  case  with 
the  standard  fruits — apples,  pears,  and  peaches. 

Domestication  of  Wild  Fruits. — Still  more  im- 
portant has  been  the  domestication  of  the  native 
fruits — grapes,  crab-apples,  gooseberries,  raspber- 


298  Plant  Life  and  Evolution 

ries,  plums,  pecans,  cranberries,  etc.  Some  of  the 
European  fruits,  as  for  instance  grapes  and  goose- 
berries, do  not  thrive  in  the  Eastern  United  States 
largely  on  account  of  diseases  to  which  they  are 
very  susceptible.  Our  native  vines  and  gooseberries, 
while  much  inferior  to  the  European  varieties,  are 
practically  immune  to  these  diseases,  and  have 
through  crossing  and  selection  given  rise  to  very 
much  improved  varieties,  which  yield  a  fairly  satis- 
factory substitute  for  the  more  tender  European 
sorts. 

Much  attention  has  been  given  of  late  years  to 
the  development  of  new  varieties  by  crossing  and 
selection.  In  this  connection  the  work  of  Luther 
Burbank  has  for  several  years  attracted  much  at- 
tention, and  deservedly  so,  although  his  work  has 
been  very  much  over-exploited  by  newspapers  and 
cheap  magazines,  in  search  of  startling  novelties. 
Burbank,  through  long  years  of  experimentation, 
combined  with  an  extraordinary  natural  gift  for 
recognizing  the  essential  characters  of  the  plant  with 
which  he  is  working,  has  been  able  to  accomplish 
what  look  like  veritable  miracles  to  the  layman. 
However,  most  of  his  results  have  been  obtained  by 
the  same  methods  of  crossing  and  selection  which 
all  plant-breeders  use.  Some  of  his  results 
are  hard  to  understand,  and  offer  some  difficult 
problems  to  the  student  of  heredity.  Through  the 
labors  of  Burbank,  and  other  plant-breeders,  many 
new  and  important  varieties  of  cultivated  plants 


Human  Factor  in  Plant  Evolution    299 

have  been  added  to  the  long  list  of  those  previously 
grown. 

Recent  Work  in  Introducing  New  Plants. — Many 
new  plants  have  been  introduced  from  foreign 
countries — hardy  wheat  and  apples  from  Russia, 
fitted  to  survive  the  severe  climates  of  the  Dakotas 
and  Montana;  dates  from  Northern  Africa  and 
Arabia  have  been  planted  successfully  in  the  hot 
deserts  of  Arizona  and  Southern  California;  and 
very  many  other  fruits  and  ornamental  plants  have 
come  from  various  parts  of  the  world.  Especially 
is  this  the  case  with  Japan,  which  has  given  to 
our  gardens  many  of  our  choicest  ornamental  plants, 
and  several  valuable  fruits.  Japanese  plants,  as  a 
rule,  are  particularly  adapted  to  the  Eastern  United 
States,  where  they  often  do  much  better  than  Eu- 
ropean plants.  The  climate  of  Pacific  Asia  is  very 
much  like  that  of  Atlantic  North  America,  and 
Asiatic  plants  find  themselves  very  much  at  home 
in  the  American  gardens. 

The  great  importance  of  the  work  of  the  United 
States  Department  of  Agriculture,  and  the  State  Ex- 
periment Stations,  in  improving  the  character  of 
agricultural  and  horticultural  products  of  the  coun- 
try, need  not  be  dwelt  upon  at  length.  By  the  study 
of  methods  of  cultivation,  and  the  improvement  and 
selection  of  varieties  adapted  to  different  parts  of 
the  country,  the  study  of  plant  diseases,  and,  last 
but  not  least,  the  introduction  of  new  varieties  from 
foreign  countries,  the  wealth  of  the  country  has 


300  Plant  Life  and  Evolution 

been  increased  by  a  sum  many  times  greater  than 
the  total  amount  spent  in  maintaining  these  experi- 
ment stations.  As  a  result  of  this  great  work,  by 
proper  selection  of  varieties  and  improved  methods 
of  agriculture — e.g.,  dry  farming  in  the  arid  West — 
the  area  of  land  fitted  for  agriculture  has  been  very 
much  extended  within  recent  years,  and  land  which 
for  ages  has  remained  a  barren  waste,  now  is  com- 
pelled to  yield  its  crops  of  grain  and  fruit  as  a 
reward  for  the  ingenuity  and  persistence  of  man. 


CHAPTER  X 
THE  ORIGIN  OF  SPECIES 

THE  mutability  of  all  organisms  is  universally 
recognized  by  modern  biologists,  and  the 
origin  of  new  types  or  "  species,"  as  the  result  of 
such  mutability,  is  no  longer  questioned.  Many 
attempts  have  been  made  to  explain  the  mechanics 
of  the  origin  of  species,  and  the  laws  governing 
them;  but  none  of  the  very  divergent  theories  pro- 
posed can  be  said  to  offer  a  satisfactory  explanation 
of  all  the  facts  concerned. 

Experimental  Morphology. — For  a  number  of 
years  the  efforts  of  many  of  the  ablest  biologists 
have  been  devoted  to  what  has  been  called  experi- 
mental morphology,  or  a  study  of  the  effects  of  vari- 
ous stimuli  upon  the  structures  of  organisms.  It 
becomes  more  and  more  evident  that  plants  are  as- 
tonishingly plastic,  and  respond  very  quickly  to 
stimuli  of  many  kinds  which  may  exercise  powerful 
formative  effects  upon  their  structures.  Plants,  be- 
ing especially  adaptable,  and  generally  more  easily 
handled  than  animals,  have  naturally  received  much 
attention  at  the  hands  of  the  experimenter,  and  the 
results  of  these  studies  have  added  much  to  our 
knowledge  of  the  laws  governing  the  development 
301 


302  Plant  Life  and  Evolution 

of  living  things.  The  very  readiness  with  which 
plants  respond  to  stimuli  is,  however,  a  source  of 
danger  in  making  sweeping  generalizations  from 
insufficient  data.  Our  ignorance  of  the  internal 
mechanism  of  the  cell,  and  the  fact  that  often  the 
full  effect  of  a  stimulus  is  not  always  immediately 
evident,  make  it  necessary  to  exercise  extreme  cau- 
tion in  explaining  the  real  significance  of  apparently 
quite  obvious  reactions  to  stimuli. 

Another  source  of  error  is  the  too  general  ap- 
plication of  results  drawn  from  a  study  of 
plants  to  the  behavior  of  animals  under  like  con- 
ditions. While  it  is  doubtless  true  that  the  proto- 
plasm of  plants  and  animals  is,  so  far  as  we 
can  judge,  very  similar  in  its  composition,  and  in 
a  general  way  reacts  in  much  the  same  manner  to 
similar  stimuli,  it  must  be  remembered  that  the  two 
kingdoms,  plants  and  animals,  have  diverged  fur- 
ther and  further  away  from  the  ancestral  organisms, 
and  this  divergence  has  resulted  in  sharply  marked 
differences,  both  structural  and  physiological,  so  that 
we  cannot  safely  argue  from  the  behavior  of  one 
of  the  higher  plants  under  certain  conditions  what 
would  be  the  result  upon  an  animal  subjected  to 
the  same  conditions.  This  can  perhaps  be  best 
shown  in  considering  the  questions  of  reproduction 
and  inheritance. 

Parallel  Development  of  Reproduction  in  Plants 
and  Animals. — Plants  and  animals  show  a  remarka- 
ble parallelism  in  the  evolution  of  the  reproductive 


The  Origin  of  Species  303 

cells  and  the  methods  of  fertilization,  and  this  is 
also  true  of  the  evolution  of  the  nuclei  of  the  body 
cells.  These  resemblances  are  all  the  more  wonder- 
ful, as  it  is  difficult  to  see  how  one  type  could  have 
been  inherited  directly  from  the  other.  At  the 
time  when  plants  and  animals  definitely  parted  com- 
pany, sexuality  was  either  not  developed  at  all  or 
was  on  a  very  low  plane,  and  consisted  in  the  sim- 
ple fusion  of  two  similar  gametes  or  sometimes  of 
two  complete  individuals.  Moreover,  the  nuclei  of 
these  primitive  organisms,  such  as  the  Flagellata, 
do  not  seem  to  possess  the  complexity  of  structure 
found  in  the  nuclei  of  the  higher  plants  and  ani- 
mals. Since  the  sexual  elements  of  the  higher  plants 
and  animals  are  independent  developments,  it  is 
probable,  in  spite  of  the  close  resemblances,  that 
there  are  inherent  differences  in  their  nature,  cor- 
responding to  the  differences  existing  in  the  bodies 
of  plants  and  animals,  and  it  is  not  likely  that  the 
laws  governing  the  development  of  one  will  apply 
without  exception  to  the  other.  We  may,  for  in- 
stance, show  that  the  early  segregation  of  the  sexual 
elements  in  many  animals  justifies  the  assumption 
of  a  special  germ  plasm ;  but  if  we  try  to  apply  this 
hypothesis  to  plants,  it  breaks  down  completely,  as 
it  is  quite  impossible  to  cite  any  evidence  for  such 
segregation  of  sexual  cells  in  these  organisms,  which 
in  very  many  cases  do  not  arise  from  sexual  cells 
at  all  and  may  be  asexual  throughout  their  whole 
existence. 


304  Plant  Life  and  Evolution 

Imperfect  Individualization  in  Plants. — One  of 
the  greatest  differences  between  the  higher  plants 
and  animals  is  the  imperfect  individuality  of  the 
former  compared  with  the  more  highly  individual- 
ized animal.  We  have  already  pointed  out  that  an 
oak  is  not  an  individual  in  the  same  sense  that  a 
dog  is.  Each  leaf-bud  of  the  tree  is  a  potential 
individual,  and  the  whole  is  a  colony  of  like  individ- 
uals, rather  than  a  single  organism.  We  may  cut 
off  a  twig  and  plant  it,  and  in  time  we  shall  have 
another  tree  with  all  its  parts,  including  flowers, 
complete.  There  is  here  no  question  of  the  develop- 
ment of  sexual  cells  from  similar  germs  in  the 
bud  from  which  the  tree  grew,  as  there  is  abso- 
lutely no  trace  of  any  flowering  structures,  and  it 
may  be  years  before  the  new  tree  is  large  enough  to 
produce  flowers.  Moreover,  the  tree  itself  is, 
properly  speaking,  asexual,  sexuality  being  re- 
stricted to  the  insignificant  gametophytes,  arising 
respectively  from  the  embryo-sac  and  the  pollen- 
spore. 

As  the  regeneration  of  the  whole  plant  is  possible 
from  a  mere  fragment  of  a  bud  or  leaf,  the  germ 
plasm,  if  it  is  present,  must  be  distributed  through- 
out the  somatic  tissues,  and  therefore  exposed 
equally  with  them  to  the  direct  action  of  external 
stimuli.  This  lack  of  individuality  and  great  power 
of  regeneration,  as  well  as  the  ready  response  to 
stimuli  of  various  kinds,  makes  it  hard  to  discrimi- 
nate between  what  may  be  considered  purely  onto- 


The  Origin  of  Species  305 

genetic  or  fluctuating  variations,  and  those  which 
may  be  assumed  to  be  of  hereditary  value. 


VARIATION 

That  all  organisms  vary  is  plain  from  the  most 
casual  study.  Not  only  are  no  two  individuals  ex- 
actly alike,  but  no  two  organs  of  an  individual  are 
identical.  Thus  it  would  be  impossible  to  pick  out 
two  leaves  or  flowers  which  are  the  same  in  all  re- 
spects. The  source  of  these  variations,  and  their 
value  in  the  evolution  of  new  species,  are  the  ques- 
tions which  are  engaging  the  attention  of  many 
biologists  at  the  present  time. 

In  attempting  to  determine  the  causes  of  these 
universal  variations,  we  at  once  meet  with  an  almost 
unsurmountable  obstacle.  It  is  practically  impossi- 
ble to  determine  to  what  degree  the  differences  be- 
tween two  plants  of  the  same  species,  growing  under 
apparently  the  same  conditions,  are  due  to  inherent 
peculiarities,  and  how  much  to  extrinsic  factors 
which  may  not  be  evident.  That  there  are  individ- 
ual idiosyncrasies  in  plants,  as  well  as  in  animals,  is 
certain.  No  two  individuals  in  a  lot  of  seedlings 
will  be  exactly  alike,  and  the  differences  may  be  very 
striking;  but  what  causes  the  apparently  greater  in- 
herent robustness,  for  example,  of  one  and  the  weak- 
ness of  another,  is  difficult  to  analyze.  Whatever 
may  have  been  the  cause  of  the  superiority  of  cer- 
tain individuals,  the  superiority  is  evident,  and  must 


306  Plant  Life  and  Evolution 

give  the  plant  a  better  chance  in  the  struggle  for 
existence.  It  does  not  necessarily  follow  that  the 
more  robust  members  of  a  lot  of  seedlings  may  be 
the  ones  that  finally  survive,  as  accidents  may  occur ; 
they  may  be  ravaged  by  insects  or  birds,  and  indeed 
might  be  preferred  by  these  to  their  less  vigorous 
competitors.  But  taking  all  things  into  account,  it 
is  reasonable  to  suppose  that  the  more  vigorous  in- 
dividuals will  in  the  end  get  the  advantage  of  their 
weaker  rivals,  and  leave  a  larger  number  of  off- 
spring to  transmit  their  more  robust  constitution. 

Continuous  and  Discontinuous  Variation. — Biol- 
ogists recognize  two  main  forms  of  variations,  small 
or  continuous  variations  forming  a  practically  un- 
broken series,  connecting  the  extremes  within  the 
species;  and  a  second  kind,  the  so-called  discon- 
tinuous variation,  where  there  is  the  sudden  ap- 
pearance of  a  character  without  intermediate 
ones  between  it  and  the  type.  Under  the  latter 
head  are  included  the  "  sports  "  of  the  gardener,  and 
the  "  mutations  "  of  De  Vries  and  his  followers. 

Fluctuating  variations  may  perhaps  best  be 
studied  in  cultivated  plants,  especially  those  which 
are  known  to  be  true  species,  where  variation  is  not 
due  to  hybridization.  A  single  example  will  suffice 
to  illustrate  this  point.  I  have  recently  had  in  my 
garden  a  large  number  of  plants  of  Cosmos,  a  fa- 
vorite autumn  flower  in  California,  and  a  good 
botanical  species.  The  plants  were  of  three  varie- 
ties, white,  pink,  and  crimson,  and  the  variations 


The  Origin  of  Species  307 

in  other  respects  were  very  striking.  In  height  the 
plants  ranged  from  three  to  ten  feet,  and  while 
these  variations  were  due  to  some  extent  to  the  dif- 
ferences in  the  soil  and  moisture,  the  difference  be- 
tween individuals  growing  together  was  sufficiently 
striking.  The  flowers  showed  very  great  differ- 
ences in  size,  in  the  form  of  the  rays,  and  in  the 
shade  of  color,  ranging  from  pure  white  through 
various  intermediate  shades  of  pink  and  red  to 
deep  crimson.  The  leaves  also  varied  greatly  in 
size,  breadth,  and  remoteness  of  their  fine  divisions. 
Similar  variations  might  be  cited  for  almost  any 
cultivated  species  grown  in  quantity,  and  to  a  less 
degree  for  most  wild  ones. 

The  gardener  by  selection  can  easily  control  any 
of  these  variations,  e.g.,  the  color  of  the  flower,  and 
it  is  probable  that  natural  selection  could  also  take 
hold  of  such  variations.  If,  for  instance,  crimson 
flowers  should  be  more  useful  than  white  ones,  it 
is  quite  conceivable  that  the  white  ones  might  be 
eliminated,  as  the  result  of  natural  selection  alone. 

The  Range  of  Fluctuating  Variations  Dependent 
upon  the  Nature  of  the  Variation. — The  limits 
within  which  fluctuating  variations  may  act  un- 
doubtedly depend  very  much  on  the  character  of 
the  variation  involved.  There  may,  for  instance,  be 
physiological  reasons  which  forbid  variations  in  cer- 
tain directions  beyond  a  given  point.  Thus,  for  ex- 
ample, by  selection  the  percentage  of  sugar  in  the 
roots  of  sugar  beets  has  been  doubled,  but  it  has 


308  Plant  Life  and  Evolution 

been  found  almost  impossible  to  increase  the  per- 
centage materially  beyond  this  point,  presumably 
due  to  the  fact  that  the  constitution  of  the  cells  will 
not  permit  of  higher  concentration  of  the  cell 
sap.  It  is  probable  that  there  are  limits  in  size  also 
beyond  which  certain  forms  cannot  go.  It  is  hardly 
likely  that  any  amount  of  selection  will  enable  the 
gardener  to  grow  pansies  as  big  as  peonies,  or 
cherries  the  size  of  oranges. 

Sports. — Discontinuous  variations  have  of  late 
attracted  much  attention,  owing  to  the  important 
work  of  De  Vries  and  his  disciples.  These  discon- 
tinuous variations,  or  "  sports,"  have  been  long 
known  and  have  given  rise  to  many  garden  varieties 
of  fruits  and  flowers.  One  of  the  best-known  cases 
is  that  of  the  nectarine,  which  is  a  sport  from  the 
peach.  The  importance  of  De  Vries'  work,  which 
will  be  referred  to  presently  somewhat  more  at 
length,  is  his  systematic  study  of  the  origin  of  these 
sudden  variations,  and  the  demonstration  that  they 
may  be  made  permanent,  where  crossing  is  pre- 
vented. 

ORTHOGENESIS 

It  is  believed  by  many  biologists  that  variation  is 
often  determinate,  i.e.,  along  definite  lines,  probably 
adaptive  in  their  nature.  If  the  definition  of  deter- 
minate variation,  "  Variation  along  special  or  par- 
ticular lines  of  adaptation,"  is  accepted,  there  can  be 


The  Origin  of  Species  309 

no  doubt  that  such  determinate  variation  is  a  com- 
mon phenomenon  among  plants.  Among  the  most 
striking  of  these  cases  are  the  several  quite  unrelated 
cases  of  sex-evolution  in  the  lower  plants,  as  well 
as  its  development  in  animals,  and  also  the  remarka- 
ble resemblances  already  referred  to  in  the  structure 
of  the  nuclei  and  the  complex  details  of  nuclear 
divisions  in  plants  and  animals.  The  striking  simi- 
larities in  the  evolution  of  the  flowers  in  mono- 
cotyledons and  dicotyledons  are  also  an  excellent 
example  of  such  determinate  variation.  The  devel- 
opment of  sympetalous  flowers  has  occurred  a  num- 
ber of  times  in  both  monocotyledons  and  dicoty- 
ledons, and  the  same  is  true  of  the  inferior  ovary, 
and  the  development  of  zygomorphic  flowers,  which 
characterize  the  most  specialized  types  in  both 
classes.  These  have  been  developed  quite  inde- 
pendently in  response  to  the  same  needs,  in  this  case 
pollination  by  insects  and  birds.  The  development 
of  special  colors  in  flowers  as  a  means  of  attracting 
birds  and  insects,  e.g.,  the  frequent  occurrence  of 
bright  scarlet  in  ornithophilous  flowers,  and  the  de- 
velopment of  nectar  and  scent,  may  very  well  be 
cited  as  examples  of  such  definite  or  determinate 
variation.  Many  other  instances  might  be  cited,  but 
one  more  will  suffice.  The  character  of  the  leaves 
of  whole  families  or  genera  is  often  as  character- 
istic as  the  flower.  In  the  vetches  we  expect  to 
find  the  tendril  always  assuming  the  place  of  the 
terminal  leaflet,  while  in  Smilax  quite  as  uniformly 


310  Plant  Life  and  Evolution 

we  find  it  developed  from  the  base  of  the  petiole. 
The  replacing  of  the  primitive  leaf  by  vertical 
phyllodes  in  many  species  of  Acacia  may  be  also 
considered  as  a  case  of  determinate  variation,  and 
the  many  more  that  might  be  cited  all  tend  to  show 
that  the  essential  organization  of  all  the  higher 
plants,  at  least,  is  sufficiently  alike  to  produce  much 
the  same  reactions  in  response  to  similar  stimuli, 
so  that  there  is  a  marked  similarity  in  the  structures 
resulting  from  adaptation  to  special  conditions  in 
various  lines  of  development. 

Variation  Greatest  in  Lately  Developed  Char- 
acters.— In  his  studies  on  variation,  Darwin  em- 
phasizes the  fact  that  variability  is  much  more 
marked  in  what  one  may  assume  to  be  later  developed 
characters.  Thus  generic  characters  are  less  varia- 
ble than  specific  ones,  and  wild  species  are  very 
much  less  variable  than  cultivated  ones.  Just  what 
constitutes  a  "  species  "  is  more  or  less  a  matter 
of  personal  opinion.  How  far  such  varieties  as 
those  described  by  De  Vries,  which  can  be  main- 
tained by  artificial  selection,  can  be  called  species, 
is  not  an  easy  matter  to  decide.  Many  cultivated 
races  of  both  plants  and  animals,  which  are  un- 
doubtedly variations  from  some  single  recognized 
species,  differ  far  more  widely  from  this  and  from 
each  other,  than  is  the  case  among  many  wild 
species,  or  sometimes  even  genera ;  but  the  purity  of 
such  artificial  species  can  be  maintained  only  by 
careful  artificial  selection.  Even  in  nature  the 


The  Origin  of  Species  311 

limits  of  species  are  often  very  difficult  to  decide, 
and  no  two  students  of  any  large  genus  of  plants 
agree  exactly  as  to  the  number  of  species  within 
the  genus.  How  these  species  have  originated  is 
a  vexed  question  which  for  the  last  generation  has 
aroused  an  amount  of  speculation,  and  even  acri- 
monious controversy,  that  would  seem  to  show  that 
men  of  science  are  not  much  behind  the  theologians 
in  their  defense  of  the  true  faith  against  the  at- 
tacks of  heretics. 

THEORIES  OF  EVOLUTION 

The  idea  of  evolution  is  a  very  ancient  one,  but 
until  the  iQth  century  evolutionary  theories  were 
too  vague  to  attract  any  general  attention.  Dar- 
win's great  service  to  science  is  not  that  he  first 
enunciated  the  principles  of  evolution,  but  that  by 
rigorous  experiment  and  observation  he  made  the 
fundamentals  of  evolution  so  clear  that  henceforth 
they  could  not  be  ignored.  Especially  did  he  make 
evident  the  enormous  importance  of  natural  selec- 
tion in  the  evolution  of  new  forms  of  life,  and  the 
origin  of  species.  As  Darwin's  studies  included 
plants  as  well  as  animals,  and  covered  an  extensive 
range  of  topics  connected  with  some  of  the  most 
important  botanical  problems,  the  debt  of  the  bot- 
anist to  Darwin  is  a  very  great  one.  The  botanical 
student  will  remember,  however,  that  during  the 
ten  years  preceding  the  appearance  of  Darwin's 


312  Plant  Life  and  Evolution 

"  Origin  of  Species,"  the  German  botanist,  Hof- 
meister,  published  a  series  of  researches  upon  the 
comparative  morphology  and  development  of  the 
higher  plants,  that  were  truly  epoch  making;  for 
while  he  did  not  expressly  enunciate  the  theory  of 
evolution,  his  whole  work  was  based  upon  the  as- 
sumption that  the  seed-plants  were  derived  from 
the  ferns,  which  in  turn  were  the  descendants  of 
moss-like  progenitors.  While  most  of  his  work  has 
been  amplified  and  corrected  in  the  light  of  new 
discoveries  and  by  the  aid  of  improved  methods  of 
research,  nevertheless  the  fundamental  principles  of 
his  work  remain  to-day  as  the  basis  of -the  com- 
parative morphology  of  the  higher  plants. 

Nee-Darwinism. — As  is  so  often  the  case,  the  dis- 
ciples have  gone  far  ahead  of  their  master  in  up- 
holding his  theories,  which  have  assumed  for  them 
the  form  of  a  veritable  dogma.  Some  of  the  fol- 
lowers of  Darwin  have  assumed  the  omnipotence 
of  natural  selection,  in  which  they  see  the  only 
factor  in  evolution,  denying  that  environment  can 
have  any  direct  effect  in  modifying  an  organism, 
or  at  any  rate  that  any  such  change  can  be  trans- 
mitted to  its  offspring.  As  every  reader  of  Darwin 
knows,  he  fully  recognized  the  importance  of  en- 
vironment as  a  formative  agency,  and  was  perfectly 
aware  of  the  value  of  much  of  the  work  of  La- 
marck, his  great  predecessor,  who  was  not  appre- 
ciated during  his  lifetime,. but  who  has  been  valiantly 
defended  by  a  host  of  ardent  advocates  during  the 


The  Origin  of  Species  313 

past  generation.  These  Neo-Lamarckians  have  ar- 
rayed themselves  against  the  Neo-Darwinians,  who 
claim  that  they,  and  they  alone,  are  truly  ortho- 
dox. Much  ingenuity  has  been  expended  in  the 
elaboration  of  theories  which  shall  prove  one  side 
or  the  other,  but  it  must  be  confessed  that  to  the 
outsider  these  theories  often  savor  more  of  meta- 
physics than  of  natural  science.  For  an  admirable 
summary  of  the  present  status  of  evolutionary  the- 
ories the  reader  may  consult  Professor  Kellogg's 
recent  work,  "  Darwinism  To-day." 

Let  us  briefly  examine  the  testimony  of  plants 
as  to  the  method  by  which  new  forms  arise, 
or  if  you  will,  the  methods  of  the  origin  of  species. 

Natural  Selection. — The  essence  of  the  Dar- 
winian theory  is  that  species  have  arisen  by  natural 
selection,  as  the  result  of  the  struggle  for  existence 
necessitated  by  the  fact  that  there  are  always  many 
more  individuals  produced  than  can  possibly  come 
to  maturity.  Darwin  believed  that  natural  selec- 
tion acted  upon  the  slight  fluctuating  variations 
which  constantly  occur  in  all  species,  but  he  also  rec- 
ognized the  possibility  of  new  species  being  started 
by  discontinuous  variations,  or  sports,  although  he 
considered  these  of  secondary  importance  in  species 
forming.  That  natural  selection  does  act  is  easily 
enough  demonstrated  by  direct  observation;  but  it 
is  not  so  easy  to  show  why  certain  individuals  sur- 
vive while  others  perish.  While  in  a  general  way 
it  may  be  assumed  that  the  fittest  survive,  this  does 


314  Plant  Life  and  Evolution 

not  by  any  means  necessarily  follow,  as  accidents 
may  result  in  the  destruction  of  the  most  vigorous 
forms  while  the  weaker  survive  to  multiply  their 
kind.  Thus  in  a  lot  of  seedlings  exposed  to  the 
attacks  of  insects,  birds,  and  other  enemies,  there 
seems  to  be  little  discrimination,  and  the  vigorous 
young  plants  are  perhaps  even  more  likely  to  suffer 
than  the  weaklings.  Darwin  assumes  that  there 
may  be  developed  as  the  result  of  natural  selection 
protective  devices,  as,  for  example,  poisonous  or  of- 
fensive secretions,  which  may  render  the  plants  dis- 
tasteful, and  of  course  it  is  the  individuals  in  which 
these  characters  are  most  pronounced  which  will 
be  most  likely  to  survive.  Unfortunately  the  dem- 
onstration of  this  hypothesis  is  by  no  means  easy. 
There  is  no  question  about  the  variability  of  all 
species.  Not  only  does  the  same  plant  vary  under 
different  conditions,  but  among  young  seedlings 
growing  under  apparently  exactly  similar  condi- 
tions a  large  amount  of  variation  can  be  observed, 
and  one  must  admit  that  the  variation  in  many  cases 
must  be  considered  as  the  result  of  individual  pe- 
culiarities, not  to  be  explained  as  the  result  of  any 
evident  environmental  factors.  Within  the  limits 
of  any  species  there  may  be  a  decided  range  of  va- 
riation, often  showing  a  wide  departure  from  what 
may  be  called  the  mode  or  type.  Such  slight 
differences  are  especially  common  in  certain  genera, 
and  are  the  source  of  endless  confusion  in  attempt- 
ing to  define  the  limits  of  species.  In  Europe  the 


The  Origin  of  Species  315 

genera  Hieracium  and  Rubus  have  been  split  up 
into  very  many  species  by  some  botanists,  while 
others  reduce  them  to  a  small  fraction  of  these,  re- 
garding the  differences  as  not  sufficiently  definite  or 
constant  to  warrant  raising  these  varying  forms  to 
specific  rank. 

In  America,  at  the  present  time,  there  is  a  strong 
tendency  to  treat  some  of  our  native  genera  in 
much  the  same  manner.  The  Thorns  (Cratsegus) 
offer  a  notorious  example  of  this.  In  the  latest 
edition  of  "  Gray's  Manual  "  sixty-five  species  are 
described,  while  in  the  edition  of  1868  but  nine 
species  were  recognized.  Unfortunately  it  is  im- 
possible to  trace  the  pedigree  of  any  of  these  forms, 
and  whether  they  have  arisen  by  the  accumulation 
of  small  differences,  or  whether  they  are  the  results 
of  discontinuous  variations  or  mutations,  we  are  not 
in  a  position  to  say. 

Artificial  Selection. — Perhaps  the  strongest  evi- 
dence offered  by  plants  for  the  truth  of  the  -Dar- 
winian theory  of  natural  selection,  is  the  origin  of 
new  forms  under  cultivation  by  selection  alone.  In 
this  way,  for  instance,  the  amount  of  sugar  in  the 
root  of  the  sugar  beet  has  been  doubled,  and  the 
percentage  of  quinine  in  the  bark  of  certain  varie- 
ties of  Cinchona  has  been  very  largely  augmented. 
Many  varieties  of  flowers  and  vegetables  have  also 
resulted  from  pure  selection.  Among  Burbank's 
many  interesting  results  is  one  which  illustrates  the 
origin  of  a  new  form  by  selection  alone.  From 


316  Plant  Life  and  Evolution 

the  vivid  orange-yellow  California  poppy  he  has  de- 
veloped a  bright  red  variety  by  selecting  individuals 
which  showed  traces  of  red  in  the  petals,  and  then 
by  successive  selection  of  the  offspring,  the  red  color 
was  intensified  until  a  pure  red  flower  resulted. 
Similar  examples  of  such  selections  can  be  shown 
in  the  development  of  many  cultivated  plants,  and 
thus  it  is  proved  that  under  certain  conditions,  at 
least,  selection  alone  is  sufficient  for  the  origin  of 
new  forms.  Whether  or  not  natural  selection  is 
the  usual  form  of  the  origin  of  new  species,  there 
can  be  little  doubt  that  it  is  an  essential  factor 
in  maintaining  the  new  form  after  it  has  once 
appeared. 

LAMARCKISM 

Modifications  in  Cultivated  Plants  Due  to 
Changed  Environment. — No  student  of  plant-be- 
havior can  doubt  that  the  structure  of  the  plant  is 
readily  affected  by  its  environment.  Changes  in  the 
environment  are  quickly  reflected  by  changes  in  the 
structure  of  the  plant.  As  we  have  already  con- 
sidered this  at  some  length  in  a  former  chapter,  the 
matter  will  not  be  enlarged  upon  here.  The  ques- 
tion naturally  arises:  Are  the  ontogenetic  changes 
due  to  enviroment  capable  of  transmission  ?  To  this 
the  experimenter  answers  at  once,  They  certainly  are. 
Striking  instances  of  this  are  the  numerous  cases 
of  acclimatization.  Races  of  domestic  plants,  e.g., 


The  Origin  of  Species  317 

corn,  from  the  Southern  States,  where  the  grow- 
ing season  is  long,  require  a  much  longer  time  to 
come  to  maturity,  and  the  seed  of  such  races  planted 
at  the  North  will  often  fail  to  mature  the  first  sea- 
son, but  if  any  of  the  seed  is  matured  and  planted 
a  second  season,  the  time  of  maturity  will  be  mark- 
edly less,  and  this  peculiarity  is  transmitted  so  that 
in  the  course  of  a  few  generations  a  race  is  devel- 
oped which  is  fitted  to  the  shorter  seasons  of  the 
North.  It  has  been  observed  that  in  the  United 
States  the  European  varieties  of  apples,  pears,  and 
some  other  fruits  have  given  place  largely  to  new 
forms  which  have  arisen  often  spontaneously,  as  the 
result  of  new  climatic  conditions  in  America. 
Moreover,  the  eastern  varieties  of  fruits  grown  in 
the  Northwest  and  on  the  Pacific  coast,  have  become 
similarly  changed,  apparently  in  response  to  dif- 
ferent climatic  conditions,  and  these  changes  are 
heritable. 

It  is  now  a  common  practice  to  grow  seeds  of 
many  vegetables  and  flowers  for  the  eastern  trade, 
under  the  very  favorable  climate  of  the  coast  valleys 
of  California,  where  the  long  dry  summers  permit 
very  perfect  maturing  of  the  seed.  It  has  been 
found,  however,  that  from  time  to  time,  seeds  grown 
in  the  East  must  be  sent  back,  otherwise  there  is  a 
tendency  to  produce  a  plant  which  will  require  for  its 
growth  a  longer  time  than  is  furnished  by  the  short 
eastern  summer.  Many  plants  in  the  California 
gardens  also  require  a  much  longer  period  of  growth 


318  Plant  Life  and  Evolution 

before  they  flower,  this  being  notably  the  case  with 
many  bulbous  plants. 

Experiments  with  Alpines. — Some  very  interest- 
ing experiments  have  been  made  by  Bonnier  and 
other  students  upon  alpine  plants,  which  illustrate 
very  clearly  the  hereditary  character  of  changes  in- 
duced by  altered  environment.  In  some  cases  a 
plant  was  divided  into  two  portions,  one  being 
grown  at  a  low  altitude,  and  the  other  transferred 
to  an  alpine  station,  where  it  was  grown  for  several 
years.  The  latter  in  time  developed  the  dwarf  habit, 
and  several  other  characteristics  of  a  true  alpine 
plant.  After  several  years,  these  artificial  alpines 
were  returned  to  the  old  environment,  and  it  was 
found  that  they  gradually  reverted  to  their  original 
form,  and  that  the  time  necessary  for  this  was  ap- 
proximately the  same  as  that  required  to  make  the 
change  to  the  alpine  type.  It  is  a  fair  assumption 
that  a  plant  transferred  to  a  new  environment,  and 
subjected  to  this  for  a  long  period,  would  to  all 
intents  and  purposes  thus  become  a  new  species,  and 
it  is  not  likely  that,  if  after  a  thousand  years  it 
were  returned  to  its  old  environment,  it  would  ever 
revert  exactly  to  its  original  condition. 

It  is  probable  that  a  critical  study  of  plants 
long  naturalized  in  a  new  country  would  show  marks 
of  constant  change  from  the  original  type.  It  would 
be  interesting  to  know,  for  example,  whether  the 
common  European  weeds  that  have  been  naturalized 
in  the  United  States  for  two  or  three  hundred  years, 


The  Origin  of  Species  319 

have  in  that  time  changed  sufficiently  to  make  them 
greatly  different  from  their  European  prototypes. 
If  such  changes  have  taken  place,  it  would  also  be 
interesting  to  know  whether  taken  back  to  the  old 
home  they  would  revert  perfectly  to  the  original 
type.  A  critical  study  of  this  kind  might  very  well 
throw  much  light  upon  the  mechanics  of  species 
making.  Bumpus  made  a  study  of  the  English 
sparrow  in  the  United  States,  and  he  found  that 
a  perceptible  modification  had  arisen  in  the  bird 
since  it  was  introduced  into  the  United  States,  some 
thirty  years  earlier.  Considering  the  readiness  with 
which  plants  vary,  and  the  very  much  longer  time 
that  many  weeds  have  been  naturalized  in  America, 
it  would  be  expected  that  a  certain  amount  of 
change  had  taken  place. 

The  conclusion  to  be  drawn  from  a  study  of  the 
behavior  of  plants  seems  to  be  that  both  fluctuating 
and  discontinuous  variations  are  important  in 
species  forming.  The  effect  of  environment  upon 
organisms  is  unmistakable,  and  these  effects  are, 
sometimes  at  least,  transmissible  to  a  greater  or 
less  extent.  This  variation  offers  the  handle  for 
natural  selection  to  grasp,  and  the  permanency  of 
any  variation  must  depend  upon  natural  selection. 

THE  MUTATION  THEORY 

The  most  notable  theory  of  evolution  that  has  ap- 
peared of  late,  is  the  Mutation  Theory  of  De  Vries. 


320  Plant  Life  and  Evolution 

His  book,  "  Die  Mutationstheorie,"  published  in 
1901,  attracted  at  once  much  attention,  and  was  a 
great  stimulus  to  the  further  study  of  variation  in 
both  plants  and  animals.  The  basis  of  this  theory 
is  that  new  species  arise,  not  by  the  accumulation  of 
slight  or  fluctuating  variations,  but  by  discontinuous 
variations  or  sudden  leaps,  which  are  immediately 
of  specific  value. 

Discontinuous  Variations :  Sports. — Reference 
has  already  been  made  to  these  discontinuous  varia- 
tions, or  sports,  of  which  many  have  been  recorded, 
especially  in  cultivated  plants.  A  list  of  the  most 
important  of  these  has  recently  been  given  by  Lotsy 
in  his  work,  "  Vorlesungen  iiber  Descendenz- 
theorien,"  1906.  The  first  of  these  recorded  muta- 
tions dates  back  to  1590,  when  there  appeared  in 
the  garden  of  the  Apothecary  Sprenger,  in  Heidel- 
berg, a  well-marked  variety  of  Chelidoninm  majus, 
which  proved  to  be  constant  and  reproduced  itself 
perfectly  from  seed,  thus  behaving  like  the  "  mu- 
tants "  of  De  Vries.  A  very  widely  cultivated  plant 
which  is  supposed  to  be  a  mutant,  is  the  well-known 
Lombardy  poplar,  which  has  been  propagated  in  a 
purely  non-sexual  way  for  a  very  long  time.  It  is 
supposed  to  be  a  sport  from  the  European  Populus 
nigra.  All  of  the  individuals  that  are  known  are 
staminate,  and  consequently  cannot  propagate  from 
seed.  The  purple-leaved,  and  cut-leaved,  varieties 
of  many  trees  and  shrubs,  are  probably  examples  of 
such  sports,  and  many  at  least  have  been  found  to 


The  Origin  of  Species  321 

reproduce  themselves  from  seed  if  care  is  taken  to 
prevent  crossing.  A  recently  described  example  of 
what  appears  to  be  the  appearance  of  a  new  species 
by  mutation,  is  that  of  Capsella  Heegeri,  described 
by  Solms-Laubach,  who  considers  it  to  be  a  new 
species  derived  by  mutation  from  the  common  shep- 
herd's-purse,  C.  bursa-pastoris. 

De  Vries'  Studies  on  the  Evening  Primrose 

The  theory  of  De  Vries  is  based  principally  upon 
a  very  extensive  series  of  studies  of  the  variation 
in  an  American  plant,  the  large  flowered  evening 
primrose,  (Enothera  Lamarckiana.  This  plant  was 
found  growing  wild  near  Amsterdam,  where  it  had 
evidently  escaped  from  cultivation,  and  an  examina- 
tion showed  that  it  was  in  a  very  variable  condition. 
For  a  series  of  years  the  plant  was  kept  under  ob- 
servation, and  many  cultures  were  made,  with  the 
result  that  certain  marked  variations  were  found  to 
appear  repeatedly,  and  these  varieties,  or  incipient 
species,  according  to  De  Vries,  were  nearly  always 
constant,  when  kept  from  crossing  with  other  va- 
rieties, or  with  the  original  type.  From  the  behavior 
of  these  "  mutants,"  as  De  Vries  called  these  va- 
rieties, he  came  to  the  conclusion  that  new  species 
always  arise  by  such  discontinuous  variations,  and 
their  persistence  as  permanent  species  depends  upon 
whether  or  not  they  survive  as  the  result  of  natural 
selection. 

De  Vries'  Theory  of  Pangenes. — De  Vries  be- 
lieves that  heredity  is  to  be  explained  by  the  assump- 


322  Plant  Life  and  Evolution 

tion  that  the  sexual  cells  contain  certain  structures, 
"  pangenes,"  which  are  the  transmitters  of  heredi- 
tary characters.  He  assumes  also  that  there  are 
certain  periods  in  the  history  of  the  species,  when  for 
some  reason  a  state  of  great  instability  and  great 
variability  prevails,  and  that  at  such  periods  muta- 
tions occur.  A  mutation  is  to  be  explained  by  the 
development  of  an  additional  one  of  the  "  heredity 
bearers  "  in  the  germ  cells  of  the  mutant,  or  by 
the  destruction  of  one  of  these ;  while  in  mere  varie- 
ties, the  number  of  these  hereditary  units  is  the 
same  as  in  the  parent  species,  but  one  or  more  of 
them  may  remain  latent.  While  such  an  explana- 
tion is  logical  enough,  it  must  be  confessed  that  it  is 
quite  incapable  of  proof,  and  may  very  well  be  re- 
garded with  some  skepticism. 

De  Vries'  experiments  on  (Enothera  Lamarckiana 
have  been  repeated  in  America,  especially  under  the 
direction  of  MacDougal  in  New  York,  and  the  re- 
sults are  entirely  in  accord  with  those  obtained  by 
De  Vries. 

Darwin  regards  the  accumulation  of  slight  fluc- 
tuating variations  as  the  basis  upon  which  natural 
selection  works,  while  De  Vries  contends  that  these 
fluctuating  variations  are  of  no  value  in  evolution, 
and  that  mutations  alone  are  important;  but  he 
agrees  with  Darwin  that  the  variation,  once  estab- 
lished, must  depend  upon  natural  selection  for  its 
maintenance.  Whether  the  mutation  theory  will 
prove  the  all-sufficient  explanation  of  the  origin  of 


The  Origin  of  Species  323 

species  is  extremely  doubtful,  but  the  clear  demon- 
stration of  the  nature  of  these  discontinuous  varia- 
tions, and  the  fact  that  they  do  offer  a  rational 
explanation  for  the  origin  of  species,  makes  the 
work  of  De  Vries  of  very  great  value. 

Criticism  of  De  Vries'  Work. — The  most  serious 
criticism  of  De  Vries'  work  is  the  uncertainty  of 
the  origin  of  the  species  upon  which  he  worked. 
CEnothera  Lamarckiana  is  not  known  at  present  in 
a  wild  condition,  and  Bateson  has  gone  so  far  as  to 
suggest  that  it  is  itself  a  hybrid,  and  recent  experi- 
ments by  Davis  indicate  a  possible  origin  of  (E. 
Lamarckiana  from  a  cross  of  CE.  biennis  and  CE. 
grandiflora.  If  such  should  be  the  case,  Bateson 
further  says  that  the  alleged  mutants  may  be  only 
reversions,  or  "  recessives,"  in  the  Mendelian  phrase- 
ology. The  occurrence  of  other  similar  mutants, 
which  are  certainly  not  of  hybrid  origin,  however, 
and  which  are  capable  of  breeding  true,  would  indi- 
cate that  the  mutants  of  De  Vries  are  not  merely 
recessive  hybrids. 

Lotsy,  in  the  work  already  referred  to,  suggests 
a  very  probable  cause  for  the  origin  of  mutants 
which  does  not  involve  the  idea  of  special  "  inheri- 
tance units."  His  view  is  that  the  tendency  to  muta- 
tion does  not  lie  in  the  egg-cell,  or  sperm,  but  arises 
subsequent  to  fertilization  as  the  result  of  the  mating 
of  the  most  dissimilar  gametes,  which  would  nat- 
urally only  occur  rarely. 


324  Plant  Life  and  Evolution 

HEREDITY 

Chromosomes  the  Bearers  of  Heredity. — The 
mystery  of  heredity  has  always  aroused  the  interest 
of  biologists,  and  many  ingenious  theories  have 
been  propounded  to  explain  it,  but  all  of  these  are 
more  or  less  unsatisfactory,  as  they  assume  premises 
which  are  impossible  of  demonstration.  The  facts 
of  fertilization,  as  they  have  been  studied  in  both 
plants  and  animals,  indicate  that  the  sexual  nuclei 
are  undoubtedly  the  most  important  parts  of  the  cell 
in  the  sexual  process,  and  Strasburger  believes  that 
in  the  higher  plants  the  chromosomes  alone  are  po- 
tent as  the  bearers  of  heredity,  since  only  the  sexual 
generative  nucleus  from  the  pollen-tube  enters  the 
egg-cell.  We  must  remember,  however,  that  in  the 
lower  plants  the  whole  protoplasts  of  the  gametes 
fuse,  cytoplasm  as  well  as  nuclei.  Moreover,  it  must 
not  be  forgotten  that  in  very  many  plants  inheri- 
tance is  through  nonsexual  methods,  principally  by 
budding,  where  there  is  no  development  of  special 
reproductive  cells  as  is  the  case  in  sexual  repro- 
duction. 

If  the  chromosomes  are  allowed  to  be  the  vehicles 
of  transmission  of  hereditary  characters,  it  still 
remains  to  be  explained  how  this  is  accomplished. 
The  assumption  that  they  contain  innumerable 
"  determinants "  which  correspond  to  the  special 
structures  of  the  adult,  is  a  simple  explanation,  but 
not  a  satisfactory  one.  De  Vries'  theory  of  "  pan- 


The  Origin  of  Species  325 

genes  "  is  more  likely  to  approximate  the  truth. 
These  ultimate  structural  units  of  the  cell  are  sup- 
posed to  be  of  relatively  few  kinds,  but  capable  of 
an  infinite  variety  of  combinations.  De  Vries  com- 
pares them  to  the  letters  of  the  alphabet,  which  are 
capable  of  combination  into  an  almost  infinite  num- 
ber of  words.  It  may  be  questioned,  however, 
whether  specific  differences  necessarily  involve  new 
kinds  of  pangenes,  as  De  Vries  believes  to  be  the 
case. 

Heredity  a  Physiological  as  well  as  a  Morpho- 
logical Question. — The  investigations  of  experi- 
mental morphology  are  making  it  more  and  more 
likely  that  the  effects  of  extrinsic  stimuli  are  potent 
in  heredity,  and  that  heredity  is  quite  as  much  a 
physiological  problem  as  a  morphological  one.  This 
view  has  been  set  forth  in  a  particularly  striking 
fashion  by  Professor  F.  Darwin,  in  his  recent  ad- 
dress as  president  of  the  British  Association  for  the 
Advancement  of  Science.  Professor  Darwin  argues 
that  the  effects  of  stimulation  may  be  cumulative, 
and  transmissible,  and  that  the  ordered  sequence  in 
the  development  of  the  individual,  "  the  rhythm  of 
ontogeny,"  is,  as  he  puts  it,  a  habit.  It  has  been 
shown  by  many  experiments,  e.g.,  those  of  Jennings 
on  the  infusoria,  that  the  effect  of  repeated  stimula- 
tion is  a  different  reaction  on  the  part  of  the  cell 
to  the  later  stimulus.  The  "  physiological  state,"  to 
use  Jennings'  phrase,  is  altered,  and  a  habit  is  es- 
tablished. 


326  Plant  Life  and  Evolution 

F.  Darwin  borrows  from  Seman  the  word 
"  engram "  to  indicate  the  effects  of  a  stimulus 
upon  the  protoplasm,  and  Darwin  argues  that  these 
engrams  are  permanent  and  transmissible  from  cell 
to  cell.  As  he  explains  it  the  engrams,  or  results 
of  stimulation,  are  of  the  nature  of  memory.  "  My 
view  is  that  the  rhythm  of  ontogeny  is  actually  and 
literally  a  habit.  It  undoubtedly  has  the  feature 
which  I  have  described  as  preeminently  character- 
istic of  habit,  viz.,  an  automatic  quality  which  is 
seen  in  the  performance  of  a  series  of  actions  in 
the  absence  of  the  complete  series  of  stimuli  to 
which  they  (the  stages  of  ontogeny)  were  originally 
due.  This  is  the  chief  point  on  which  I  wish  to 
insist:  It  means  that  the  resemblance  between  on- 
togeny and  habit  is  not  merely  superficial,  but  deeply 
seated.  ...  It  cannot  be  denied  that  the  onto- 
genetic  rhythm  has  the  two  qualities  observable  in 
habit — namely,  a  certain  degree  of  fixity  or  auto- 
maticity,  and  also  a  certain  variability.  It  is  not 
irrevocably  fixed,  but  may  be  altered  in  various 
ways.  Parts  of  it  may  be  forgotten  or  new  links 
may  be  added  to  it.  In  ontogeny  the  fixity  is  espe- 
cially observable  in  the  earlier,  the  variability  in 
the  later,  stages." 

Darwin's  mnemic  theory  does  not  involve  the 
assumption  of  special  determinants  or  pangenes,  and 
as  we  have  already  pointed  out  in  Chapter  II,  the 
similarity  in  the  constitution  of  the  egg,  and  the  sub- 
jection of  the  developing  embryo  to  practically  iden- 


The  Origin  of  Species  327 

tical  conditions,  are  sufficient  to  account  for  the 
principal  phenomenon  of  hereditary  transmission 
without  the  assumption  of  the  presence  of  a  special 
germ  plasm.  This  view  frankly  admits  that  the  de- 
velopment of  the  organism  not  only  may  be  directly 
influenced  by  external  factors,  but  that  the  changes 
so  induced  may  be  inherited. 

Mendel's  Law  of  Heredity. — Some  40  years  ago 
a  German  monk,  Gregor  Mendel,  published  in  the 
proceedings  of  an  obscure  scientific  society  the  re- 
sults of  a  series  of  experiments  upon  the  laws  of 
heredity.  These  were  quite  ignored  by  the  scientific 
world  until  attention  was  called  to  them  by  De 
Vries  and  some  other  experimenters,  almost  simulta- 
neously, in  connection  with  the  revived  interest  in 
the  study  of  heredity,  aroused  by  De  Vries'  work 
on  mutation.  The  great  value  of  Mendel's  work 
lies  in  its  showing  that  there  are  definite  laws  gov- 
erning the  inheritance  of  certain  characters.  Men- 
del worked  especially  upon  varieties  of  garden  peas, 
and  demonstrated  that  where  any  two  contrasting 
characters  were  crossed,  as  for  instance  long  stems 
and  short  stems,  smooth  and  wrinkled  seeds,  that 
these  characters  were  inherited  in  definite  propor- 
tions, and  one  of  the  characters  was  "  dominant  " 
while  the  other  was  "  recessive."  For  instance, 
when  a  variety  with  round  seeds  is  crossed  with  one 
producing  wrinkled  ones,  when  these  are  self-fer- 
tilized, the  progeny  will  all  produce  seeds  of  the 
round  type,  which  is  thus  shown  to  be  dominant. 


328  Plant  Life  and  Evolution 

The  plants  resulting  from  the  round  seeds  of  the 
second  generation,  however,  are  not  all  alike,  but 
some  will  produce  round  seeds  while  others  show 
the  wrinkled  seeds,  and  the  proportion  in  which 
these  are  produced  will  be  three  plants  with  round 
seeds  to  one  plant  with  the  wrinkled  seeds.  The 
offspring  of  the  latter,  or  recessive  type,  will  breed 
true,  but  of  the  three-quarters  with  the  round  seeds 
only  one-quarter  are  pure  dominants,  breeding  true, 
while  the  offspring  of  the  other  half  are  hybrids, 
dividing  in  the  next  generation  in  the  same  ratio, 
three  dominants  to  one  recessive,  and  so  on.  This 
behavior  implies  that  the  sex  cells  are  either  pure 
dominants  or  pure  recessives,  and  when  fusing  in 
crossing  will  produce  either  pure  dominants,  re- 
cessives, or  hybrids  between  the  two,  the  proportion 
following  closely  the  law  of  probability. 

Professor  Bailey  explains  the  law  as  follows: 
"  Differentiating  characters  in  plants  reappear  in 
their  purity  and  in  mathematical  regularity  in  the 
second  and  succeeding  hybrid  offspring  of  these 
plants;  the  mathematical  law  is  that  each  character 
separates  in  each  of  these  generations  in  one-quarter 
of  the  progeny  and  thereafter  remains  true.  In 
concise  figures  it  is  expressed  as  follows :  iD  :  2DR  : 
iD.  iD  and  iR  continuing  true,  but  DR  breaks 
up  again  into  the  dominants  and  recessives  in  the 
ratio  of  three  to  one." 

The  Mendelian  law  has  been  applied  to  a  good 
many  cases  of  inheritance  in  both  plants  and  ani- 


The  Origin  of  Species  329 

mals,  and  has  proved  true  in  many  instances,  al- 
though there  are  numerous  exceptions  for  which 
as  yet  there  is  not  an  adequate  explanation. 

THE  EVOLUTION  OF  SEX 

The  characters  of  the  sexual  cells  in  plants  and 
animals  often  show  extraordinary  similarity,  and  the 
details  of  fertilization  are  very  much  the  same.  On 
account  of  these  similarities  it  is  commonly  assumed 
that  the  nature  of  sexuality  in  plants  and  animals 
is  identical,  but  a  careful  examination  of  the  phe- 
nomena connected  with  the  sexual  process  in  plants 
and  animals  will  show  differences  which  are  suf- 
ficiently striking  to  make  one  cautious  in  drawing 
too  comprehensive  conclusions  as  to  the  nature  of 
sexual  reproduction  in  general. 

Evolution  of  Sex  in  Plants  and  Animals  Not 
Identical. — Although  the  nature  of  the  sexual  cells 
in  the  higher  plants  and  animals  is  apparently  so 
much  alike,  the  history  of  the  evolution  of  sex  in 
plants  and  animals  has  been  apparently  quite  dif- 
ferent. In  animals,  sex  is  far  more  important  than 
in  plants,  and  seems  to  have  been  established  once 
for  all.  All  Metazoa  are  sexual  at  some  stage  of 
their  development,  and  there  is  no  existing  evidence 
of  that  transition  from  the  non-sexual  condition  to 
the  sexual  which  is  shown  so  clearly  in  several  evo- 
lutionary lines  in  the  plant  kingdom.  Between  con- 
jugation of  two  individuals  found  in  the  Infusoria, 


330  Plant  Life  and  Evolution 

and  the  clearly  differentiated  eggs  and  sperms  of 
the  lower  Metazoa  there  are  no  intermediate  stages. 
With  the  increasing  individuality  shown  by  the 
higher  animal  types,  the  sexual  elements  become 
more  and  more  sharply  segregated  from  the  somatic 
tissues,  and  the  direct  derivation  cf  the  generative 
tissues  of  one  generation  from  those  of  the  pre- 
ceding one  looks  very  plausible,  and  the  theory  of 
a  definite  germ  plasm  handed  on  from  one  genera- 
tion to  another  has  a  basis  of  actual  observation. 

Sex  has  Arisen  Repeatedly  in  Plants. — Sex  in 
plants  has  evidently  arisen  over  and  over  again. 
In  some  half  a  dozen  quite  unrelated  groups  among 
living  plants,  every  stage  of  development  of  the 
gametes  can  still  be  found,  from  nearly  or  quite 
similar  isogametes,  to  perfectly  differentiated  eggs 
and  sperms.  It  is  also  clear  that  the  simpler  types 
of  gametes  are  derived  from  non-sexual  zoospores, 
or  from  non-sexual  individuals  in  the  case  of  uni- 
cellular plants.  The  slight  difference  between  the 
sexual  and  non-sexual  cells  is  shown  by  the  ease 
with  which  some  of  the  lower  plants  may  be  forced 
to  produce  one  or  the  other  as  the  result  of  differ- 
ent stimuli.  Klebs'  experiments  upon  various  algse 
and  fungi  are  especially  striking  as  showing  how 
readily  the  character  of  the  reproductive  cells  may 
be  controlled. 

While  the  mechanism  of  fertilization  may  be  very 
much  the  same  in  plants  and  animals,  the  results 
are  usually  very  different.  The  fusion  of  the  egg- 


The  Origin  of  Species  331 

cell  and  sperm  is  effected  much  in  the  same  way, 
but  the  history  of  the  zygote  in  animals  and  plants 
is,  as  a  rule,  not  at  all  alike.  In  the  animals  the 
egg  develops  at  once  into  the  embryo,  which  sooner 
or  later,  either  directly  or  after  a  metamorphosis, 
becomes  a  single  individual  like  the  parent.  This 
is  rarely  the  case  in  plants.  In  the  green  algse,  the 
zygote  usually  becomes  a  resting  spore,  whose  main 
function  is  to  carry  the  plant  over  periods  of  stress. 
Only  rarely,  and  this  is  especially  the  case  in  such 
marine  types  as  Fucus,  does  the  zygote  develop  at 
once  into  the  definite  plant.  In  the  majority  of 
the  fresh-water  algae,  as  we  have  seen,  the  develop- 
ment proceeds  only  after  a  period  of  rest,  and 
though  the  zygote  may  germinate  directly  into  a 
new  plant,  much  more  commonly  it  first  divides  into 
a  number  of  free  cells,  each  of  which  gives  rise 
to  a  new  individual,  and  this  interpolation  of  a  neu- 
tral stage  between  the  zygote  and  the  production 
of  new  sexual  plants,  becomes  more  pronounced  in 
the  higher  plants. 

While  the  alternation  of  generations,  which  is 
so  conspicuous  in  all  green  plants  above  the  algae, 
is  sometimes  met  with  in  animals,  as  in  some 
insects  and  hydroids,  it  is  far  less  common  than 
in  plants.  In  all  of  the  higher  plants,  from  the 
ferns  to  the  seed-plants,  it  must  be  remembered 
that  the  predominant  phase  is  the  non-sexual 
sporophyte.  Much  confusion  has  arisen  from  over- 
looking this  fact.  The  sexual  cells  of  a  flowering 


332  Plant  Life  and  Evolution 

plant  are  not  parts  of  the  sporophyte  at  all, 
but  belong  to  the  insignificant  sexual  generation, 
or  gametophyte,  included  within  the  ovule,  or  devel- 
oped from  the  pollen-spore  during  its  germination. 
The  embryo  develops,  not  into  a  gametophyte  like 
the  plant  which  produces  the  egg,  but  into  a  sporo- 
phyte, which  produces  non-sexually  myriads  of 
spores,  embryo-sacs  or  pollen-spores,  which  in  turn 
develop  the  gametophytes.  A  single  fertilization, 
therefore,  results  ultimately  in  an  enormous  number 
of  new  gametophytes,  although  years  may  elapse 
before  the  sporophyte  becomes  large  enough  to 
flower  and  produce  its  crop  of  spores.  Now  to  as- 
sume that  there  is  a  special  germ-plasm,  which  is 
passed  on  from  the  tiny  gametophyte  to  the  non- 
sexual  and  long-lived  sporophyte,  and  finally  segre- 
gated in  the  spores,  and  again  passed  along  to  the 
next  generation  of  gametophytes  is,  to  say  the  least, 
improbable. 

Subordination  of  Sex  in  the  Higher  Plants — 
The  relative  unimportance  of  sex  in  plants  is  shown 
by  the  predominance  of  the  asexual  condition  in  all 
of  the  higher  plants.  It  is  in  the  more  primitive 
aquatic  forms,  like  the  algae  or  the  amphibious 
archegoniates,  that  sexuality  is  best  developed,  and 
it  is  evident  that  this  is  directly  associated  with 
their  aquatic  life,  as  in  these  plants  the  sperms  are 
motile  and  require  water  for  their  transport.  In 
the  more  highly  developed  land  plants  sexuality  be- 
comes more  and  more  subordinated,  and  not  in- 


The  Origin  of  Species  333 

frequently  may  be  entirely  suppressed,  the  sporo- 
phyte  multiplying  itself  solely  by  vegetative  division. 
Some  seaweeds  and  many  fungi  appear  to  be  also 
entirely  sexless,  and  it  is  hard  to  see  wherein  these 
plants  are  inferior  in  size  or  structure  to  the  sexual 
types.  This  subordination  of  the  sexual  conditions 
in  so  many  plants  is  in  striking  contrast  with  the 
universal  occurrence  of  sexuality  in  the  Metazoa. 
It  may  be  safely  asserted  that  the  substitution  of 
asexual  reproduction  for  the  sexual  method  in 
plants  is  largely  due  to  the  generalized  character  of 
their  tissues,  the  cells  being  far  more  plastic  than 
those  of  the  more  highly  specialized  animals,  and 
therefore  capable  of  an  almost  unlimited  degree  of 
regeneration. 

THE  SIGNIFICANCE  OF  SEX 

The  underlying  reasons  for  the  development  of 
sex  have  been  the  subject  of  endless  speculation,  but 
there  is  no  general  agreement  among  biologists  as  to 
what  these  causes  are.  Most  of  these  speculations 
have  been  from  the  zoological  side,  and  it  is  at 
least  doubtful  whether  they  will  apply  equally  to 
plants,  where  sexual  differentiation  might  very  well 
have  arisen  in  response  to  quite  different  causes.  In 
animals  it  has  been  assumed  that  there  is  a  "  physio- 
logical need  "  for  fertilization,  but  just  what  this 
is,  is  not  explained.  Fertilization,  or  the  union  of 
two  gametes,  has  been  considered  advantagebus  both 


334  Plant  Life  and  Evolution 

as  an  inducement  for  variation,  and  for  the  preser- 
vation of  hereditary  characters;  but  even  among 
animals,  as  Kellogg  has  shown  from  his  studies  on 
both  bees  and  aphides,  the  individuals  produced  by 
parthenogenesis  are  quite  as  variable  as  those  devel- 
oped from  fertilized  ova. 

In  following  the  history  of  the  lower  plants,  it 
is  very  evident  that  the  result  of  fertilization,  what- 
ever may  have  inaugurated  it,  is  twofold :  first,  the 
development  of  a  resting  stage  for  carrying  the  plant 
over  unfavorable  conditions  of  drought  or  cold ;  and 
second  the  increased  production  of  new  plants,  as 
each  zygote  usually  produces  more  than  one  plant 
on  germination.  The  second  cause  is  probably  the 
more  important  factor  in  the  evolution  of  sex  in 
plants,  and  it  has  been  closely  associated  with  the 
production  of  the  resting  stage,  or  terrestrial  phase, 
in  the  green  algae  and  their  descendants,  the  arche- 
goniates;  but  in  the  strictly  aquatic  seaweeds,  the 
rapid  multiplication  by  zoospores  is  quite  sufficient, 
as  no  resting  stage  is  necessary,  and  where  an  elab- 
orate sexual  system  is  present,  as  in  the  red  algae, 
the  same  object  is  attained  by  the  development  of 
a  sporophyte  from  which  numerous  spores  are  pro- 
duced without  any  resting  stage. 

The  better  development  of  the  sexual  cells  in  the 
fresh-water  green  algae  when  compared  with  their 
marine  relations,  as  well  as  the  usual  low  grade  of 
sexuality,  or  its  complete  absence,  in  the  brown  algae, 
and  certain  green  seaweeds,  implies  that  there  is 


The  Origin  of  Species  335 

some  connection  between  the  conditions  of  life  in 
fresh  water  and  the  evolution  of  sexual  cells.  This 
may,  perhaps,  be  sought  in  the  necessity  for  most 
fresh-water  plants  to  provide  resting  stages.  The 
zygote  resulting  from  the  union  of  two  small,  similar 
gametes  is  provided  with  a  smaller  amount  of  food 
material  than  is  the  case  where  there  are  well-de- 
veloped sexual  cells,  large  eggs  and  small  sperms. 
Moreover,  the  larger  amount  of  contents  in  such  a 
large  spore  allows  for  a  greater  number  of  spores 
on  germination,  and  so  might  be  an  advantage. 
Whether  the  fusion  of  the  gametes  provides  a 
greater  store  of  energy  as  well,  can  hardly  be 
proven,  but  it  is  not  at  all  unlikely.  It  is  thus  quite 
conceivable  that  the  need  for  a  resting  stage  fitted 
to  produce  quickly  a  number  of  germs  at  the  end 
of  the  dormant  period,  was  the  most  important 
factor  in  the  specialization  of  the  sexual  cells  in 
the  fresh-water  algae.  The  futher  evolution  of  the 
zygote,  as  shown  in  the  highly  complex  sporophyte 
of  the  terrestrial  plants,  has  been  sufficiently  dwelt 
upon  in  an  earlier  chapter.  The  absence  of  any 
need  for  the  terrestrial  resting  stage  in  the  green 
and  brown  seaweeds  may,  perhaps,  explain  the  gen- 
erally low  type  of  sexual  cells  in  these  forms. 

The  peculiar  type  of  reproduction  in  the  red  algae 
has  evidently  arisen  quite  independently,  and  the 
peculiar  spore  fruit  has  its  nearest  analogy  in  that 
of  certain  fungi.  It  will  be  noted  that  in  the  red 
algae  also,  the  fertilized  cell  does  not  produce  new 


336  Plant  Life  and  Evolution 

plants  like  the  parent,  but  a  sporophyte  bearing  many 
asexual  spores,  so  that,  as  in  the  green  algae,  a  single 
fertilization  results  in  the  production  of  many  new 
individuals. 


HYBRIDIZATION 

Natural  Hybrids. — It  was  long  the  general  be- 
lief that  true  hybrids  were  necessarily  sterile,  but 
experiment  has  shown  that  hybrids  may  be  quite  as 
fertile  as  the  parent  species,  or  even  in  some  cases 
may  surpass  them  in  fertility.  This  at  once  opens 
up  the  question  whether  new  species  may  not  some- 
times arise  immediately  as  the  result  of  crossing  of 
two  other  species,  and  there  is  abundant  evidence 
that  this  is  sometimes  the  case.  While  hybrids  are 
usually  of  rare  occurrence  in  nature,  there  are  many 
records  of  such,  and  in  some  cases  they  occur  in 
numbers  equal  to  the  parent  species,  and  are  appar- 
ently quite  as  well  fitted  to  survive.  These  natural 
hybrids  have  been  much  more  carefully  studied  in 
Europe  than  in  America,  where  the  number  of  au- 
thentic cases  is  relatively  small.  Kerner  estimates 
that  about  one  thousand  natural  hybrids  have  been 
found  in  Europe,  but  of  these  only  a  small  number 
have  survived  and  perpetuated  themselves.  As  these 
latter  may  be  perfectly  fertile  and  apparently  fitted 
to  their  environment,  it  is  hard  to  see  why  they 
should  not  be  considered  as  good  species.  Most  of 
these  were  described  as  valid  species  before  their 


The  Origin  of  Species  337 

hybrid  nature  was  recognized,  and  some  of  them 
have  been  repeatedly  produced  by  artificial  cross- 
ing of  the  parent  species.  Narcissus  poeticus 
crossed  with  the  daffodil  (N.  pseudo  narcissus) 
produces  N.  incomparabilis,  a  favorite  garden 
form  which,  however,  has  also  been  found  repeat- 
edly occurring  wild  where  the  two  parent  species 
grow  near  each  other.  N.  odorus  has  also  been 
shown  to  be  a  cross  between  N.  poeticus  and  N.  jon- 
quilla.  A  species  of  Foxglove,  Digitalis  purpuras- 
cens,  is  a  cross  between  D.  purpurea  and  D.  lutea, 
and  the  hybrid  alpine  rose,  Rhododendron  inter- 
medium, of  the  Tyrol,  grows  with  its  parents,  R. 
ferrugineum  and  R.  hirsutum,  being  sometimes 
more  abundant  than  either  of  the  parent  forms. 

One  of  the  most  remarkable  of  the  hybrids  is  an 
orchid,  Nigritella  suaveolens,  which  often  occurs 
in  large  numbers  in  the  alpine  meadows,  and  has 
been  shown  to  be  a  cross  between  N.  angustifolia 
and  Gymnadenia  conopsea,  belonging  to  a  different 
genus.  The  orchids,  however,  are  often  more  fer- 
tile with  pollen  from  other  species  or  even  other 
genera,  and  many  bi-generic  hybrids  are  known  to 
the  florist.  It  is  even  possible  for  plants  belonging 
to  different  families  to  cross.  A  hybrid  has  been 
described  (see  Vernon,  "  Variations  of  Plants  and 
Animals,"  page  166)  between  Digitalis  ambigua 
(Scrophulariacea)  and  Sinningia  speciosa  (Ges~ 
neracecu). 

In  the  United  States  a  number  of  hybrids  have 


338  Plant  Life  and  Evolution 

been  described  among  the  willows,  oaks,  and 
sedges,  and  probably  many  more  remain  to  be  re- 
corded. Of  the  oak  hybrids  may  be  mentioned  that 
between  the  white  oak  and  the  bur  oak,  and  a  species 
described  as  Qnercus  brittoni  which  was  shown  to 
be  a  cross  between  Q.  Marylandica  and  Q.  ilicifolia. 
A  fern,  Asplenium  ebenoidcs,  is  supposed  to  be 
a  hybrid  between  the  walking  fern  (Camptosorus 
rhizophyllus)  and  Asplenium  ebeneum. 

Artificial  Hybrids. — The  first  artificial  plant 
hybrids  of  which  there  is  a  definite  record  was  ob- 
tained in  1760  by  Kolreuter,  whose  further  work  in 
hybridization  was  very  important,  although  not 
fully  appreciated  by  his  contemporaries.  His  first 
hybrid  was  obtained  by  crossing  two  species  of  to- 
bacco, Nicotiana  rnstica  and  N.  paniciilata.  Later 
he  made  experiments  with  many  other  genera,  and 
demonstrated  most  of  the  important  phenomena  con- 
nected with  the  crossing  of  plants.  Subsequently 
a  long  controversy  arose  over  the  question  of  the 
sterility  of  hybrids  compared  with  species,  which 
lasted  for  a  long  time.  It  is  now  clear  that  while 
as  a  rule  sterility  results  from  crossing  widely  dif- 
ferent forms,  e.g.,  different  genera,  this  is  not  nec- 
essarily the  case,  and  fertility  or  sterility  of  the  off- 
spring cannot  be  taken  as  the  test  for  the  validity  of 
a  species.  The  importance  of  hybridization  in  caus- 
ing plants  to  vary  has  been  taken  advantage  of  by 
the  plant-breeder,  some  of  whom,  like  Burbank,  rely 
frequently  upon  such  crossing,  not  only  to  unite  cer- 


The  Origin  of  Species  339 

tain  desired  qualities,  but  also  to  disturb  the  equilib- 
rium of  the  species  and  induce  wider  varia- 
tion, which  may  thus  be  taken  as  the  basis  for 
selection. 

Aberrant  Hybrids. — Hybrids  between  two  dis- 
tinct species  are  usually  intermediate  in  character 
between  the  parents,  but  this  is  not  always  the  case. 
Sometimes  one  or  the  other  parent  is  prepotent, 
or  certain  parts  may  be  inherited  from  one  and 
some  from  the  other.  Thus  the  flower  of  the  hybrid 
may  resemble  one  parent,  and  the  leaves  the  other. 
Sometimes,  however,  the  results  of  hybridization 
are  most  unexpected,  and  the  hybrid  differs  mark- 
edly from  both  parents.  This  is  the  case  in  the 
Primus  blackberry,  produced  by  Burbank  from  the 
crossing  of  a  wild  California  species,  Rubus  ur sinus 
and  R.  cratccgifolius  from  Siberia.  The  hybrid 
differs  strikingly  from  either  parent,  and  comes  true 
from  seed,  and  might  very  well  be  described  as  an 
entirely  new  species.  Burbank  has  obtained  similar 
results  in  hybrid  walnuts,  produced  by  crossing  the 
native  California  walnut  with  the  English  walnut 
and  the  black  walnut  of  the  Eastern  States.  In 
every  case  the  hybrid  was  distinguished  by  its  ex- 
traordinary vigor,  growing  with  wonderful  rapidity, 
and  far  surpassing  either  of  the  parents  in  this 
respect.  Moreover,  in  many  cases  the  nuts  produced 
by  the  hybrid  were  much  larger  than  those  of  either 
parent.  A  cross  made  by  Burbank  between  a  yellow 
and  a  white  poppy  resulted  in  a  flame-colored  flower, 


340  Plant  Life  and  Evolution 

quite  unlike  either  of  the  parents.  The  latter  hy- 
brid Burbank  considers  to  be  a  case  of  reversion, 
but  this,  however,  may  be  questioned. 

Thus  we  see  that  as  the  result  of  crossing  be- 
tween markedly  different  forms,  quite  new  types  of 
flowers  and  fruits  have  been  developed.  Many  arti- 
ficial hybrids  of  orchids  have  resulted  from  cross- 
ing widely  separate  species,  or  even  genera,  and 
many  of  the  novelties  offered  from  time  to  time  by 
the  florists  are  hybrids,  sometimes  combining  char- 
acters derived  from  several  species.  An  example  of 
one  of  these  compound  hybrids  is  the  Shasta  daisy, 
which  is  one  of  Burbank's  "  creations."  The 
"  Plum-cot  "  is  a  cross  between  the  plum  and  apricot, 
and  the  Loganberry,  now  one  of  the  standard  fruits 
of  California,  resulted  from  crossing  a  native  black- 
berry with  the  red  raspberry.  Among  the  most 
interesting  of  the  recent  hybrid  fruits  are  Webber's 
Citrus-hybrids.  Among  these  is  the  "  Citrange," 
a  hybrid  between  the  hardy  Japanese  Citrus  trifoli- 
ata  and  the  sweet  orange,  which  it  is  hoped  may 
prove  the  beginning  of  a  race  of  hardy  sweet 
oranges.  Another  interesting  hybrid  is  the  "  Tan- 
gelo,"  a  cross  between  the  grape-fruit  or  pomelo, 
and  the  Tangerine  orange. 

The  plant-breeder  takes  advantage  of  the  insta- 
bility produced  by  changed  environment,  but  since 
the  time  of  Kolreuter's  early  experiments  in  hybrid- 
ization, the  great  importance  of  crossing  different 
forms  to  induce  variability  has  been  clearly  recog- 


The  Origin  of  Species  341 

nized.  Kolreuter  emphasized  the  importance  of 
crossing,  and  also  enunciated  the  principle  that 
variability  was  very  much  increased  by  crossing 
hybrids,  either  with  each  other,  or  with  the  parent 
form. 

Graft  Hybrids. — There  has  been  much  contro- 
versy as  to  the  possibility  of  hybrids  arising  from 
grafting.  Several  cultivated  forms  are  alleged  to 
have  so  arisen,  but  further  attempts  to  reproduce 
them  in  this  manner  have  been  unsuccessful,  and 
the  question  is  still  open  whether  the  best-known 
cases  of  such  alleged  graft  hybrids,  the  Cytisus 
adami  and  the  Crataegomespilus,  are  not  really 
hybrids  of  the  usual  type. 

That  hybrid  grafts  are  possible  has  recently  been 
shown  by  Winkler  ("  Berichte  der  Deutschen  Bo- 
tanischen  Gesellschaft,"  1907),  who  succeeded  in 
producing  an  unmistakable  hybrid  by  grafting  a 
nightshade,  Solamim  nigrum,  upon  a  tomato.  After 
the  nightshade  graft  had  united  with  the  stock,  the 
latter  was  cut  off  so  as  to  expose  a  flat  surface, 
which  included  the  united  tissues  of  the  scion  and 
stock.  From  this  cut  surface  there  arose  numerous 
adventitious  buds,  one  of  which  developed  into  a 
shoot  which  was  a  compound  of  the  tomato  and 
nightshade.  It  was  a  "  mosaic,"  one  half  being 
tomato,  the  other  half  nightshade,  and  some  of  the 
leaves  were  intermediate.  Winkler  proposed  the 
name  "  Chimsera  "  for  such  vegetable  monsters  as 
this.  Later  experiments  resulted  in  the  production 


342  Plant  Life  and  Evolution 

of  true  graft  hybrids,  which  were  almost  exactly 
intermediate  in  all  respects  between  the  nightshade 
and  tomato. 


EXPERIMENTAL  MORPHOLOGY 

Much  attention  has  been  given  of  late  to  the  ex- 
perimental study  of  the  formative  effects  of  environ- 
ment upon  the  developing  organism.  Both  zool- 
ogists and  botanists  have  become  much  interested 
in  this  question,  and  many  important  works  have 
appeared  during  the  past  few  years.  While  caution 
is  necessary  in  deducing  from  the  results  of  these 
artificial  experiments,  the  laws  governing  the  devel- 
opment of  organisms  under  normal  conditions,  nev- 
ertheless much  light  has  been  thrown,  by  these 
experiments,  upon  some  of  the  fundamental  prob- 
lems of  evolution. 

One  fact  stands  out  especially  prominent,  namely 
the  remarkable  plasticity  of  the  plant-organism, 
which  responds  readily  to  a  very  great  variety  of 
stimuli  and  shows  an  extraordinary  range  of  varia- 
tion within  the  species.  These  experimental  studies 
also  demonstrate  very  forcibly  the  generalized  char- 
acter of  the  plant  tissues,  and  the  readiness  with 
which  one  organ  may  take  over  the  functions  of 
another  when  it  is  necessary.  Space  forbids  more 
than  a  very  brief  reference  to  a  few  typical  in- 
stances. 

The  leaves  of  many  plants,  e.g.,  roses,  peas,  are 


The  Origin  of  Species  343 

provided  with  stipules,  insignificant  leaf-like  ap- 
pendages of  the  leaf-base.  If  the  blade  of  leaf  is 
removed,  these  stipules  will  often  become  very  much 
enlarged  and  take  over  the  duties  of  the  destroyed 
leaf  blade.  A  similar  change  into  a  flat  blade  has 
been  observed  in  the  slender  tendrils  which  termi- 
nate the  leaf  in  certain  plants,  like  the  pea,  when  the 
leaflets  have  been  removed.  In  some  species  of 
ferns,  the  spore-bearing  leaves  are  much  smaller 
than  the  large  sterile  leaves,  and  normally  develop 
comparatively  little  green  tissue.  If,  however,  the 
sterile  leaves,  which  are  the  principal  photosynthetic 
organs,  are  removed  while  young,  the  sporophylls 
which  arise  later  will  assume  more  or  less  perfectly 
the  character  of  the  amputated  fronds.  By  cutting 
back  the  leafy  stem  of  a  potato  plant,  the  under- 
ground shoots  which  normally  develop  into  the  tu- 
bers, will  appear  above  the  ground  and  develop  into 
leafy  shoots,  while  if  the  young  tubers  are  constantly 
removed,  the  reserve  food  which  would  normally 
be  stored  up  in  these  will  accumulate  in  tubers 
formed  above  the  ground  from  some  of  the  aerial 
shoots.  A  familiar  case  of  substitution  is  that  oc- 
curring in  many  conifers.  If  the  leading  shoot  of 
a  pine  or  fir  is  destroyed,  a  lateral  shoot  below  it 
will  usually  grow  upward  and  take  its  place.  This 
accelerated  growth  and  change  of  position  are  sup- 
posed to  be  due  to  the  diverting  of  the  flow  of 
nutritive  matter  from  the  destroyed  apical  shoots, 
to  one  of  the  lateral  ones. 


344  Plant  Life  and  Evolution 

Klebs'  Experiments  on  Formative  Effects  of 
Stimuli. — The  various  formative  factors,  light,  heat, 
food,  etc.,  have  already  been  sufficiently  discussed 
and  will  not  be  dwelt  upon  here  at  length,  but  it 
may  be  worth  while  to  refer  briefly  to  a  few  cases 
where  the  formative  effects  of  some  of  these  stimuli 
have  been  critically  studied.  Klebs  for  several  years 
has  been  investigating  the  direct  causes  affecting 
the  character  of  plant  structures,  and  the  results  of 
his  studies  are  extremely  interesting  and  valuable. 
Especially  instructive  are  some  of  his  studies  upon 
the  lower  organisms,  whose  simplicity  makes  it 
easier  to  judge  the  direct  effects  of  the  stimuli  em- 
ployed in  the  experiment.  Klebs  showed  that  it  is 
possible  for  the  experimenter  to  control  almost  ab- 
solutely the  character  of  the  development  of  the 
plant.  By  the  employment  of  certain  stimuli,  e.g., 
light  of  varying  intensity  or  color,  nutritive  media 
of  different  kinds,  etc.,  the  plant  can  be  forced  to 
develop  in  almost  any  way  the  experimenter  may 
select. 

Reproduction  of  any  type  can  be  induced,  or  the 
reproductive  activity  may  be  entirely  suppressed. 
Klebs  has  later  extended  his  studies  to  the  higher 
plants,  where,  owing  to  the  greater  complexity 
of  the  organism,  the  formative  factors  are  not 
nearly  so  clearly  evident.  Nevertheless,  in  these 
higher  plants,  also,  he  has  shown  that  it  is  possible 
to  control  to  an  extraordinary  degree  the  develop- 
ment. By  varying  the  character  of  the  light,  tern- 


The  Origin  of  Species  345 

perature,  and  nutritive  conditions,  he  showed  that 
not  only  the  habit  of  the  plant  might  be  greatly 
changed  but  the  color  and  size  of  the  flower,  and 
sometimes  even  the  number  of  different  parts,  might 
be  altered  very  much ;  so  that  in  the  case  of  a  species 
of  house-leek  (Sempervivum),  for  example,  the 
differences  in  the  flowers  were  much  greater  than 
those  between  some  of  the  species  of  the  genus. 

Klebs'  conclusions  from  his  studies  are  that  varia- 
bility and  inheritance  are  the  results  of  physiological 
changes  entirely  and  there  is  no  necessity  for  as- 
suming the  existence  of  definite  formative  structures 
or  protoplasmic  units,  pangenes,  or  determinants, 
etc.  Unfortunately  the  possibility  of  hereditary 
transmission  of  the  changes  induced  by  environ- 
mental conditions,  has  not  yet  been  investigated  as 
fully  as  could  be  wished. 

CONCLUSION 

No  Single  Theory  Satisfactorily  Explains  All  the 
Facts  of  Evolution. — From  a  study  of  the  behavior 
of  plants  in  a  state  of  nature,  as  well  as  under  ex- 
perimental conditions,  it  is  certain  that  no  one  of 
the  many  theories  that  have  been  advanced  can 
explain  satisfactorily  all  of  the  phenomena  asso- 
ciated with  the  evolution  of  the  vegetable  kingdom. 
There  is  no  question  that  mutations  or  discon- 
tinuous variations  do  occur,  and  that  these  may  be 
the  beginning  of  new  species  is  exceedingly  proba- 


346  Plant  Life  and  Evolution 

ble ;  but  that  there  is  necessarily  a  radical  difference 
between  mutations  and  fluctuating  variations  has 
not  been  satisfactorily  proven;  and  the  fact  that  by 
artificial  selection  of  slight  differences  new  forms 
may  arise,  makes  it  highly  probable  that  these  fluctu- 
ating variations  may  also  be  potent  in  species  form- 
ing under  natural  conditions.  The  occurrence  of 
well-established  natural  hybrids  makes  it  practically 
certain  that  new  species  sometimes  arise  directly  by 
the  crossing  of  two  well-marked  species. 

It  also  becomes  more  and  more  evident  that  the 
plant  organism  is  extremely  plastic,  and  readily  in- 
fluenced by  changes  in  the  environment,  and  that  the 
results  of  such  changes  may  be  transmitted  to  the 
offspring.  The  generalized  character  of  the  tissues 
of  even  the  highest  plants,  shown  especially  by  the 
study  of  regeneration,  does  not  support  the  theory 
of  a  special  germ-plasm,  directly  associated  with 
the  transmission  of  hereditary  characters.  The  view 
that  the  laws  of  heredity  are  exclusively  physio- 
logical is  probably  too  extreme,  but  on  the  other 
hand  it  does  not  seem  necessary  to  assume  the  pres- 
ence of  an  infinity  of  morphological  units,  "gem- 
mules,"  "determinants,"  etc.  It  is  almost  certain 
that  the  protoplast  does  contain  many  permanent, 
but  invisible,  organs,  comparable  to  the  nucleus  and 
chromatophores,  but  the  development  of  the  organ- 
ism probably  depends  quite  as  much  upon  the  po- 
tentialities of  these  to  respond  to  stimuli,  as  to  their 
actual  form  or  chemical  structure.  Whatever 


The  Origin  of  Species  347 

factors  may  be  shown  to  cause  the  appearance  of 
new  forms,  or  incipient  species,  the  survival  of  these 
must  depend  upon  natural  selection. 

In  the  history  of  both  the  vegetable  and  animal 
kingdoms,  the  most  important  event  was  the  desert- 
ing of  the  primitive  aquatic  environment  for  life 
in  the  air.  Fungi,  Ferns,  and  Seed-plants  on  the 
one  hand ;  Insects,  Birds,  and  Mammals  on  the  other, 
prove  the  superiority  of  the  land  over  the  sea,  as 
a  field  for  the  work  of  evolution. 


INDEX 

(Asterisks  preceding  page  numbers  denote  illustrations) 


Abronia,  203 

Acacia,  200,  207,  221,  222, 
253,  3io 

Adaptation,  17,  18,  148 

Adaptability  of  Angiosperms, 
181 

Adder-tongue  Fern  (see 
"  Ophioglossum  ") 

Aerating  organs,  196 

Agathis,  131,  141,  246 

Agave,  200,  249 

Agriculture,  Department  of, 
299,  300;  origin  of,  279, 
287 

Air-plants  (see  also  epi- 
phyte"), 168 

Alfalfa,  284 

Algas  (see  also  "  Sea- 
weeds"), 31,  54,  55,  66,  186, 
187,  194,  252,  333 

Alga  fungi  (see  also  "  Phy- 
comycetes"),  73,  74 

Alisma,  *I74 

Aloe,  180,  235,  249 

Alpine  plants,  260,  263;  arti- 
ficial, 318 

Alternation  of  Generations, 
69,  88,  112,  113,  331 

Altingia  excelsa,  261 

Amaryllis,  A.  family,  166 

Ampelopsis,  257 

Amphisporangiate  flowers, 
154,  161,  162 

Anabsena,  218 

Anemone,  227,  254,  269 

Angiosperms,  112,  132,  137, 
138,  147,  148,  149,  162,  183, 
184,  185,  240,  241,  347; 


fossil,  149,  240,  241;  origin 

of,  162,  183 
Antheridium,  87,  99 
Anthoceros,         Anthocerotes, 

*93,  94,  95,  96,  97,  *IO2,  103, 

in,  114,  115 

AntS,   221,   222 

Apetalse,  171,  177 

Apetalous  flowers,  155,  165 

Aphanochaete,  *66 

Aphyllon,  217 

Apocarpous  flowers,  155,  156 

Apogamy,  112,  113 

Apophysis,  94 

Apple,  283,  295,  297,  317 

Aquatics,  40,  41,  117,  193,  195, 

196 
Araucaria,  HI,  131,  139,  140, 

141,  246,  248,  253 
Arbutus    (see    "  Madrono  ") 
Archegoniates        (see        also 

"Mosses,"    "Ferns"),    86, 

334 

Archegpnium,  87,  99 

Arenaria  Grcenlandica,  263 

Arethusa,  *I74,  246 

Arrow-head  (see  also  "  Sa- 
gittaria"),  155,  167 

Artificial  selection,  315 

Arum,  Aroid,  161,  163,  164, 
165,  167,  245,  253 

Ascogonium,  76 

Ascomycetes,  75 

Ascus,  76 

Asexual  reproduction,  Asex- 
ual plants,  20,  21,  68,  304, 
331,  333 

Asparagus,  210 


349 


350 


Index 


Asplenium,  A.  ebeneum,  338; 

A.  ebenoides,  338 
Aster,  251 
Azalea,  246,  270 
Azolla,  195,  218 

Bacteria,  4,  5,  12,  13,  45,  48, 

50,  186,  208,  218 
Bald  Cypress   (see  "  Taxodi- 

um  ") 

Balsam   (Impatiens),  262 
Banana,  20,  245,  253,  286,  288 
Barberry  family,  257 
Barley,  281 
Barriers  to  plant  distribution, 

267 

Basidiomycetes,  75,  76 
Beech,  213,  244 
Beech-drops        (Epiphegus), 

217 
Bees,  as  agents  in  pollination, 

179 

Begonia,  113 
Benthamia,  258 
Bigeneric  hybrids,  337,  340 
Bignonia  family,  228 
Big-tree      (see      also      "Se- 
quoia"), 256 
Biophores,  7,  10,  21 
Birch,  246,  250 
Birds,  177,  179,  181,  234,  266 
Bitter-sweet   (Celastrus),  270 
Black-knot         (Plowrightia) 

212 

Bladder-kelp,  58 
Bladder-weed     (Utricularia), 

169,  219 
Blasia,  218 
Blood-root  (Sanguinaria), 

269 
Blue-green    Algae     (see    also 

"Cyanophyceae"),    49,    81 

186,  265 

Bodo  caudatus,  *5i 
Botrychium,  *IO2 
Botrydium,  *82,  83 
Bracken-fern,   118,   130 
Bramble  (see  also  "  Rubus  "), 

261,  262 


Bread-fruit,  20,  279,  286 

Broom,  210,  251 

Brown  Algae  (see  also 
"  Kelp,"  "  Phaeophyceas  "), 
59,  60,  61,  62,  66,  69,  70, 
188,  189,  190 

Bryophytes  (see  also  "  Liver- 
worts," "  Mosses"),  89 

Buckeye   (^sculus),  38,  274 

Bud-variation,  20 

Bulbs,  Bulbous  plants,  202, 
215,  249 

Bur-clover  (Medicago),  275 

Burdock,  173,  267,  288 

Bur-marigold  (Bidens),  173 

Bur-oak,  338 

Bur-reed  (Sparganium),  154, 
165,  *i67 

Buttercup,  150,  155,  161,  163, 
172,  227,  229,  254,  261,  262, 
267,  276,  290 

Butterwort  (Pinguicula),  219 

Cabomba,  196 

Cacti,  199,  201,  202,  210,  221, 

244,  249,  271,  272 
Cakile,  203 
California,  32,  249,  274;  flora 

of,  249,  274 
California       poppy       (Esch- 

scholtzia),  316 
Calochortus  (see  also  "  Mari- 

posa"),  202 
Calyx,  155 
Cambium,  159,  170 
Camptosorus,  338 
Canker-root  (Aphyllon),  217 
Canna,     Canna     family,     167, 

176,  228,  234,  253 
Cape  Region  of  Africa,  249 
Capsella,    C.    bursa   pastoris, 

321 ;  C.  Heegeri,  321 
Carboniferous     plants,      116, 

130,  131,  132,  134,  135,  236, 

238 
Cardinal  flower  (Lobelia  car- 

dinalis),  *I7I,  293 
Carex,  *I74 


Index 


Carnivorous  plants,  219 

Carpel,  152 

Casuarina,  246,  265 

Castilleia,  217 

Catalpa,  158 

Cattail    (Typha),   165,   195 

Caulerpa,  68 

Cecropia,  231 

Cedar,  274 

Cedar   apple    (Gymnosporan- 

gium),  77 
Celastrus,  270 
Cell,  9,  *io,  14,  28 
Century      plant       (see      also 

"  Agave  " ) ,    200,    202,    249, 

272 

Cercis,  270 
Cereals,  284 
Cereus  giganteus,  200 
Chaparral,  274 
Characeae,  Charales,  69,  204 
Chelidonium  majus,  320 
Chemical       constituents       of 

plants,  44 
Chemotaxis,  44,  45 
Chestnut,  261 
Chimaera,  341 
Chlamydomonas,  *5O 
Chlorophyceae        (see        also 

"Green  Algae"),  56 
Chlorophyll,   12,  208;  in  ani- 
mals, 12 
Chloroplast,     movements    of, 

28 

Choripetalse,  172,  173,  176 
Chromatophore       (see      also 

"Chloroplast"),  9,    10,    29, 

203,  204 
Chromosome,   10,  21,  324;  in 

heredity,  324 
Cinchona,  315 

Cinnyris,  Cinnyridae,  234,  235 
Citrange,  340 
Citrus,    Citrus    hybrids,    340; 

C.  trifoliata,  340 
Claytonia,  *iji,  269 
Clematis,  177,  270 


Climate,  as  factor  in  evolu- 
tion, 32,  33 

Climbing  plants,  207,  244 

Clover,  284 

Club-mosses  (see  also  "  Ly- 
copods,"  "  Lycopodium  "  ) , 
104,  105,  106,  no,  in,  115, 
116,  118,  130,  131,  135,  139, 
146;  fossil,  no 

Coal-measures,  flora  of,  116, 
146 

Cocoanut,  180,  265,  279,  283 

Coffee,  282 

Cold,  endurance  of  by  plants, 

Coleochaete,  91 

Color  of  flowers,  177,  228 

Columbine,  234 

Column  (of  Orchids),  168 

Composite,     173,     176,     226, 

227 
Comparative    morphology,    a 

clue  to  relationships,  47 
Conducting    tissue    of    pistil, 

15.6 
Conifers,      Coniferales,      no, 

139,  140,  141,  142,  143,  145, 

146,    240,    241,    273;    fossil, 

143,  240;   of  Pacific  slope, 

273 

Coprinus,  38 
Corals,  14 

Coral-root  (Corallorhiza),2i7 
Coral  tree  (Erythrina),  180 
Cordaitales,     130,     135,     140, 

146,  160,  241 
Corolla,  155 
Cosmos,  306 
Cotton,   283 
Cottonwood,  270 
Cotyledon,  159 
Crab-apple,  270,  283,  297 
Cranberry,  2/9,  298 
Crataegomespilus,  341 
Crataegus,  315 

Creosote  bush    (Larrea),  221 
Cream    cup     (Platystemon), 

275 


352 


Index 


Cretaceous,  flora  of,  150,  241, 

242,  253,  254,  259 
Cross-pollination,      144,     148, 

178,   183,  225,  227,  234,  309 
Cup-fungi,  76 
Custard    apple     family,    269, 

283 

Cutleria,  *66 
Cyanophyceae         (see        also 

"Blue-green  Algse"),  49 
Cycad,    Cycadales,    no,    in, 

126,  132,  133,  152,  160,  162, 

183,  240,  241 ;  fossil,  240 
Cycas,    *I24,    *I27,    136,    138, 

140,   141,   145 ;  C.  revoluta, 

136 

Cycadeoideae,  137 
Cypress,  142,  145,  196,  256 
Cytisus  Adami,  341 

Daffodil,  159,  251,  337 

Daisy,  267,  290 

Dandelion,  158,  173,  246,  267, 

288,  200 
Dansea,  *o8 
Darwinism,  311 
Datura,  229 
Deciduous  habit,  38 
Desmids,  203 
Desert  plants,  37,  199 
Determinant,  21,  324,  346 
Determinate     variation,     174, 

309,  3io 
Devil's     apron      (see     Lami- 

naria) 
Devonian     plants,     116,     118, 

130,  236 
Diatoms,  62,  63 
Diclinous    flowers,    154,    156, 

171,  226 
Dicotyledons,    150,    156,    159, 

161,  162,  168,  169,  170,  173, 

184,  309 

Digitalis  (see  also  "  Fox- 
glove"), D.  ambigua,  337; 
D.  lutea,  337;  D.  purpuras- 
cens,  337;  D.  purpurea, 
337 


Dionsea,  219 

Discontinuous  variation,  306, 

319,  320 

Dodder  (Cuscuta),  13,  216 
Dogwood,   177,  270 
Dogtooth    violet    (Erythroni- 

um),  269 
Dominant    (Mendelian),    327, 

328 

Dracaena,  164 
Draparnaldia,  *s6 
Drosera,  219 
Drought,    protection    against, 

43 

Dryas,  263 

Duckweed   (Lemna),  195 
Dumortiera,  239 
Durian,  283 

Ectocarpus,   *66 
Eel-grass  (Zostera),  195 
Egg,  21,  54,  67 
Elm,  244,  251,  268 
Embryo,  88,  128,  157,  161,  164 
Embryo-sac,    153,    154 
Endophyte,   106 
Endosperm,  128,  157 
Engram,  326 
Environment,   25 
Enzyme,   71,   212 
Ephedra,  143,  144 
Epigynous  flowers,  176 
Epiphegus,  217 
Epiphyte,  117,  207,  208 
Equisetum,    Equisetinese,    29, 

103,  *ios,  132 
Erodium,  29,  214 
Erythrina,  180,  235 
Erythronium,  269 
Eschscholtzia,  275 
Eucalyptus,  177,  207,  210 
Euglena,   12 
Eumycetes,   74 
Euphorbia,  199,  249 
Eusporangiate  ferns,  239 
Evening  primrose,  321 
Everlasting  flower    (Gnapha- 

litim),  262 


Index 


353 


Evolution,   theories   of,   311; 

of  sex,  66,  3*9,  329 
Exoascus,  76 
Experimental        morphology, 

48,   301,   342,   343 

Factors  in  Evolution,  19 

Ferments,  71,  212,  219 

Ferns,  79,  83,  87,  97,  103, 
116,  120,  122,  131,  208,  252; 
fossil,  116 

Fertilization,  67,  88,  128, 
156 

Fibro-vascular  system  of  tis- 
sues, 100 

Figwort   family,  217 

Field  Horsetail,  118 

Filamentous  Algae,  31 

P"ilmy  Ferns,   117 

Fir,  123,  246,  250,  269,  274 

Fission  plants  (Schizophyta), 
50 

Flagellata,  50,  51,  54,  63,  66, 
303 

Flax,   175,  281,   282,  283,  284 

Floral  envelopes,  155 

Flower,  123,  137,  148,  151, 
152,  154,  165,  170,  184 

Flowering  plants  (see  "  Seed- 
plant,"  "  Spermatophyte") 

Flowering  Dogwood,  258 

Fluctuating  variations,  23, 
305,  306,  319 

Forage  plants,  284 

Fossil  record,  46,  236 

Foxglove  (Digitalis),  176, 
177,  227,  251,  337 

Fox-grape,  258 

Fouquiera,  200 

Fresh-water  Algae,  64,  81,  87 ; 
Red,  64 

Fritillaria,  274 

Fruits,  148,  158,  180,  286;  edi- 
ble, 286 

Fucaceae,  191 

Fuchsia,   176,  234 

Fucus,  62,  *66,  189,  331;  F, 
vesiculosus,  189 


Fungi,    13,   70,   71,   208,   211, 
212,  214,  224,  333 

Gametes,  54,  67,  88,  223 
Gametophyte,   88,   89,   98,   99, 

106,  121,  133,  152,  153,  194; 

Angiosperms,    152;     Cycas, 

i'33;    Ferns,   98,   99;    Lyco- 

podium,  106 
Gaultheria,  262 
Gedeh,  flora  of,  261 
Gelsemium,  270 
Gemmules,  7,  346 
Gentian,  251,  261 
Geranium,  175,  230,  242 
Gerardia,  217 
Germ-plasm,  21,  23,  303,  330, 

346 

Gesneraceae,  337 
Giant  Cactus,  200 
Giant  Kelp,  59,  67,  80,  190 
Gilia,  275 

Ginger  family,  245,  253 
Ginkgo,  Ginkgoales,   no,  in, 

126,     131,     132,     138,     145, 

241 

Glacial  epoch,  249,  255 
Gladiolus,  *i67 
Glossopteris,  240 
Gnaphalium,  262 
Gnetales,  133,  143,  146,  152 
Gnetum,  143,  144,  152,  154 
Goldenrod,  251 
Gooseberry,  297,  298 
Gordonia,  262 
Gorse,  291 
Graft-hybrids,  341 
Grape,  270,  297,  298 
Grasses,  220,  226,  271,  284 
Gravity,  effects  of,  43 
Green   Algae,    57,   66,   69,   70, 

80,  91,  120,  190 
Greenland  Sandwort,  263 
Grevillea,  247 
Ground  Pine    (Lycopodium), 

104 

Great  Plains,  flora  of,  270 
Guava,  288 


354 


Index 


Gulf-weed  (Sargassum),  *35, 

60 
Gum  (see  also  "  Eucalyptus," 

"  Liquidambar,"  "  Nyssa  "), 

251,  256,  269 
Gunnera,  153 

Gymnadenia,  G.  conopsea,  337 
Gymnosporangium,  77 
Gymnosperms,  132,  134,  162 

Halophytes,  202 

Hamamelis,  257 

Hawaiian  Islands,  flora  of, 
263 

Hawk-moths,  179,  229 

Hawk- weed  (Hieracium), 
315 

Hawthorn,  270 

Heath  family,  217,  251 

Hemlock,  244,  269 

Hemp,  283 

Hepaticae  (see  "  Liver- 
worts ") 

Heredity,  19,  21,  22,  324,  325, 
326;  theories  of,  21 

Hermaphrodite  (see  "  Am- 
phisporangiate  ") 

Hetercecism,  77 

Heterospory,  106,  107,  108, 
122 

Heterostylism,  230 

Hofmeister,  Studies  on  Evo- 
lution, 312 

Honey  guides,  228 

Honeysuckle,  179 

Honeysucker,  179,  234 

Horned  Liverworts  (see 
"  Anthoceros,"  "  Antho- 
cerotes  ") 

Horsetails  (see  also  "  Equi- 
setum"),  103,  104,  105,  no, 
115,  116,  122,  130,  131,  132 

House-leek     (Sempervivum), 


:ucki 


Huckleberry,     47,     213,     262, 

279 
Humming-birds,       179,      220, 

234 


Hyacinth,  166 

Hybrids,  Hj&ridization,  296, 
336,  337,  338,  339 ;  Aberrant, 
339 ;  Natural,  336,  337 

Hydra,  12 

Hydroids,  14,  336 

Ice-plant       (Mesembryanthe- 

mum),  203 

Immobility  of  plants,  14 
Indian  Corn,  37,  282  (see  also 

"  Maize  ") 
Indian     Pipe      (Monotropa), 

13,  47,  79,  213,  217 
Individuality  of  plants,  304 
Indo-Malayan  flora.  247 
Infusoria,  27,  51,  325,  329 
Insects,  agents  in  pollination, 

177,  178,  224,  225,  228 
Ippmaea,  253 

Iris  family,  166,  176,  249 
Irritability  of  Protoplasm,  25 
Island  floras,  263 
Isocarpous  flowers,  175 
Isoetes,  124,  162 

Jamaica,  flora  of,  244 
Jasmine     (Gelsemium),     270 
Jeffersonia,  257 
Jute,  281 

Kauai,  flora  of,  264 

Kauri    Pine    (Agathis),    141, 

246 
Kelp      (see      also      "  Brown 

Algae"),  35,  59,  62,  188 
Krakatau,  flora  of,  264 

Labiate  flowers,  177 
Lamarckism,  316 
Laminaria,  58 
Land    plants,    origin    of,    80, 

82,  83,  84,  192,  193 
Lantana,  291 
Larch.   141,  251 
Laurel,    249,   274 
Leaf,  31,  35,  36,  37,  47,  100, 

170,  204 
Leaf-curl  of  peach,  76 


Index 


355 


Lemna,   195 
Lepidocarpon,   no,   131 
Lepidodendron,  131,  139 
Lichens,  78,  208,  213,  217,  218, 

262 

Life,  origin  of,  3,  6,  186 
Light,  effects  of,  33,  34,  203, 

205,  206,  208,  209 
Lily  family,  249,  254 
Lily-pf-the-valley,  166 
Liquidambar,  261 
Liriodendron,  242,  249 
Live-oak,  274 
Liverworts,    79,    85,    86,    89, 

121,  218,  239 
Lobelia,  171 

Loblolly  bay  (Gordonia),  262 
Locust,  269 
Loganberry,  340 
Lombardy  poplar,  320 
Lupin,  275,  276 
Lycopods,    Lycopodinese    (see 

also  "Club  moss"),   140 
Lycopodium,    104,    *IO5,    114, 

*I27 

Lysichiton,  274 

Macrocystis,  59,  60 

Macrosporangium,  125,  136 

Macrospore,  107 

Madrono  (Arbutus),  274 

Magnolia,  155,  161,  162,  163, 
172,  227,  229,  242,  249,  251, 
256,  257,  269 

Mahogany,  269 

Maiden-hair  tree  (see  "  Gink- 
go") 

Maize,  285,  286 

Mandrake  (Podophyllum), 
251,  257 

Mango,  206,  283,  288 

Mangosteen,  283 

Mangrove,  169,  196 

Manila  hemp,  281 

Manzanita  (Arctostaphylos), 
207,  210,  274 

Maple,  242,  244,  246,  268,  274, 
292 


Marattiaceae,  134,  239 

Marine  plants,  70 

Mariposa  lily  (Calochortus), 
202,  275 

Marsilia,  107 

Marsiliaceae,  131 

Mayflower  (Epigaea)  (see 
'Trailing  Arbutus") 

Mechanical  tissues,  89 

Megaceros,  *gS 

Megaspore  (see  "  Macro- 
spore  ") 

Memory,  a  factor  in  heredity, 
27,  326 

Mendel's  law  of  inheritance, 
327 

Mesembryanthemum,  203 

Mesophyte,  195,  197 

Mesozoic,  flora  of,  135,  136, 
140,  145,  149,  182,  240 

Mesquit,  244 

Metazoa,  329,  330,  333 

Micellae,  7 

Microsporangium,  125,  152 

Microspore,  107 

Milkweed  (Asclepias),  2=51 

Millet,  281 

Mint  family,  228 

Mnemic  theory  of  inheri- 
tance, 326 

Monoclea,  239 

Monocotyledons,  150,  152, 
156,  159,  161,  162,  163,  164, 
165,  173,  184,  309 

Monosporangiate  flowers,  162 

Monterey  cypress,  142 

Monterey  pine,  142 

Morus  (see  "Mulberry") 

Mosses,  87,  go,  120,  121,  194, 
208,  251 

Movements  of  plants,  28,  29 

Mulberry,  83,  *I74 

Multicellular  plants,  30,  55 

Mushroom,  75,  76 

Mustard,  37,  159,  275 

Mutation,  Mutation  theory, 
23,  306,  308,  319,  321,  322, 
323 


356 


Index 


Mycelium,  75 
Myrmecophily,  221,  222 

Narcissus,   166,   176,  337;   N. 

incomparabilis,      337 ;      N. 

jonquilla,  337;   N.   odorus, 

337 ;     N.    pseudo-narcissus, 

337 

Nasturtium,  230,  *23i 
Natural     selection,     46,    312, 

313,  347 
Nectarine,  308 
Nemophila,  275 
Neo-Darwinism,  312 
Neo-Lamarckism,  312 
Nepenthes,  219 
Nereocystis,  59,  60 
New  Zealand  flax    (Phormi- 

um),  278 
Nigritella,     N.     angustifolia, 

337;  N.  suaveolens,  337 
Nitrogen,  71 

Nitrogen-bacteria,  5,  6,  212 
Nocturnal  flowers,  229 
Norfolk   Island  pine    (Arau- 

caria),  140 
North  Temperate  Zone,  flora 

of,  250 
Nostoc,  218 
Nucleus,  9 
Nuphar,  40 

Nutrition  of  green  plants,  12 
Nutrition  of  Fungi,  77 
Nyssa,  261 

Oak  (see  also  "Quercus"), 
171,  172,  226,  242,  246,  251, 
252,  254,  268,  270,  274,  292, 
338 

CEdogonium,  *56,  *66 

OEnothera,  321,  323 ;  CE.  bien- 
tiis,  323;  CE.  grandiflora, 
323;  CE.  Lamarckiana,  321, 
3p3 

Onion  mildew,  212 

Ontogeny,  24 

Ontogenetic  variations,  24,  32, 
316;  transmission  of,  316 


Ophioglossum,  *iO2,  103,  114, 

US 

Opium,   282 
Orange,  288 

Origin  of  land  plants,  80 
Origin  of  species,  301,  311 
Orchids,  79,  167,  168,  176, 

227,  228,  231,  245,  247,  293, 

340;  hybrid,  340 
Orchis,    232;    O.    spectabilis, 

*232 

Ornithophily,     ornithophilous 

flowers,  234,  235,  309 
Orthocarpus,  275 
Orthogenesis,  308 
Ovum  (see  "Egg") 
Ovule,  123,  136,  152 

Pacific       slope,       flora       of, 

272 
Paleozoic,  flora  of,    105,    no, 

145,  237,  238 
Palms,  152,  164,  165,  245,  249, 

269,  270,  272 

Pandanus        (see       "  Screw- 
pine") 
Pangenes,    10,    21,    321,    322, 

324,  325 
Parasites,    71,    72,    211,    212, 

215,  217 

Parthenogenesis,  46,  224 
Partridge-berry     (MitchellaV 

230 

Passion-flower,  270 
Paw-paw  (Asimina),  269 
Pea-family,  175 
Peach,  297 
Pear,  297,  317 
Pecan,  298 
Pelargonium,  *23i 
Penicillium,  39 
Pentstemon,  177 
Pepperidge  (Nyssa),  261 
Pepper  family,  150,  161,    163, 

245 

Peperomia,  153 
Peridineae,  62 
Permian,  flora  of,  133,  239 


Index 


357 


Perfect  flowers  (see  "  Am- 
phisporangiate  ") 

Perfume  in  flowers,  177,  228 

Persimmon,  269,  279 

Petunia,  229 

Phaeophycese  (see  also 
"Brown  Algae"),  58,  63 

Phalloidese,  225 

Phanerogams  (see  "  Seed- 
plants") 

Photosynthesis,  5,  13,  29,  30, 
34,  55,  56,  67,  94,  188,  203, 
205 

Phyllodia,  200,  207 

Phycomycetes  (see  "  Alga- 
fungi") 

Physiological  factors  in  he- 
redity, 22 

Pine,  123,  *I24,  125,  126,  244, 
246,  252,  269,  292 

Pineapple,    Pineapple    family, 

20,    247,    269 

Pinguicula  (see  "  Butter- 
wort ") 

Pink,  172 

Pitcher-plant  (Nepenthes, 
Sarracenia),  219,  245,  293 

Pistil,  156 

Plane,  160,  242,  254 

Plankton,  63 

Plant-corals,  58 

Plants  and  animals  compared, 

II,    12 

Plant-breeding,  294,  297,  298 
Plasmodium,  27 
Platystemon,  275 
Pleurococcus,  *55 
Plum,  298 
Plum-cot,  340 
Podostemonacese,   169 
Pogonia,  258 
Poison  ivy   (Rhus),  258 
Poison  oak  (Rhus),  274 
Pollen,  125,  126 
Pollen-sac,  123,  152 
Pollen-tube,  125 
Pollinium,  232 
Polysiphonia,  189 


Pond-lily  (see  "Nuphar") 

Pond-weed,   161,   163 

Pond-scum  (Spirogyra),  55, 
203 

Poplar,  Populus,  113,  154,  160, 
171,  172,  242,  250,  254;  P. 
nigra,  320 

Poppy  (see  "  Eschscholtzia") 

Porella,  *93 

Portulaca,  172 

Postelsia,  59,  ="191 

Potato,  286 

Prairies,  flora  of,  260 

Prevention  of  self-pollina- 
tion, 230 

Prickly  pear,  201 

Primrose  (see  "  Primula  ") 

Primula  imperialis,  262 

Primus  blackberry,  339 

Pronuba  yuccasella,  233 

Protococcus,  81 

Proteacese,  246,  248,  253 

Protective  devices,  against 
animals,  227 ;  against  ex- 
cessive light,  206,  210 

Protoplasm,  4,  7,  8,  39;  struc- 
ture of,  8;  water  in,  39 

Pseudo-bulbs,  208 

Pteridophytes  (see  also 
"Ferns"),  97,  100,  105,  113, 
160,  162,  194,  238,  239;  fos- 
sil, 105 

Pteridosperms,  116,  131,  137, 
162,  239,  241 

Puff-ball,  74 

Quercus,  Q.  Brittoni,  338 ;  Q. 

Hid  folia,    338;     Q.    Mary- 

landica,  338 
Quinine,  282 

Rafflesia,  182,  216 
Ragweed,  290 
Ramie,  281 
Ranales,  163 

Ranunculus      (see     "  Butter- 
cup") 
Raspberry,  283,  297 


3*8 


Index 


Recessive    (Mendelian),   327, 

328 

Red  Algae  (see  also  "  Rhodo- 
phycese"),    58,   63,   64,    68, 
69,    80,    335;    pigments    of, 
63;  reproduction  of,  335 
Red-bud  (Cercis),  270 
Redwood  (Sequoia),  256,  274 
Regeneration  in  plants,  15,  16, 

304 
Reproduction,  20,  62,  64,  65, 

222,      223,      224,      302,      332; 

Brown  Algae,  62;  Red  Al- 
gae, 64;  nature  of  reproduc- 
tion in  the  higher  plants, 
332 

Resurrection-plant  (See  "  Se- 
laginella  ") 

Rhiphidium,  *73 

Rhizoid,  101 

Rhodophyceae  (see  also  "Red 
Algae"),  58 

Rhododendron,  47,  213,  217, 
246,  262,  270,  337;  hybrid, 
337 

Riccia,  *87,  91,  92,  *Q3 

Ricciocarpus,  *85,  86 

Rock-weed  (Fucus),  58,  189 

Rocky  Mountains,  influence 
on  climate,  259 

Roots,  86,  89,  loo,  101 

Root  stock,  215 

Rose-mildew,  72,  75 

Rubus  (see  also  "  Bramble  "), 
315,  339;  R-  cratfcgifolius, 
339 ;  R.  ur sinus,  339 

Rue-anemone,  *I74 

Sac-fungi,  75 
Sagittaria,  40,  167 
Sage-brush,  221,  271 
Sago-palm  (see  "Cycas") 
Salinity,  effects  of,  189 
Salt-marsh  plants,  202 
Salviniaceae,  131 
Sand-verbena  (Abronia),  203 
Saprolegnia,  *74 
Saprophytes,  72,  211,  215,  217 


Sarcodes,  217 

Sargassum,  *35,  60 

Sarracenia,  219 

Sassafras,  242,  249,  251,  255, 

269 

Scarlet  balm  (Monarda),  179 
Scarlet  sage,  179,  234 
Schizophyta,  50 
Scouring    rush    (see    "  Equi- 

setum  ") 
Screw-pine   (Pandanus),   153, 

154,  164,  165,  247,  265 
Scrophulariacea?,  337 
Sea-lettuce  (Ulva),  34,  57 
Sea-palm       (Postelsia),      60, 

*i9i,  192 

Sea-rocket  (see  "  Cakile  ") 
Sea-weeds,    58,   69,    187,    188, 

204,  334 
Secondary     thickening,     105, 

106 

Sedges,  174,  338;  hybrid,  338 
Seeds,  108,  109,  no,  in,  122, 

128;  origin  of,  108,  109 
Seed-ferns   (see  also  "  Pteri- 

dosperms"),  no,  115 
Seed-plants  (Spermato- 

phytes),  no,   120,   123,   128, 

130,  145,  146;  fossil,  145 
Selaginella,  104,  105,  109,  123, 

124,  125,  126,  129;  S.  lepido- 

phylla,  42 
Selection,  factor  in  evolution, 

46,  307 

Sempervivum,  345 
Senecip,  247 
Sensitive      fern      (Onoclea), 

258 

Sequoia,  142,  242,  249,  255 
Sex,    Sexuality,    75,    86,    154, 

190,  214,  329,  330,  332,  333 ; 

significance  of,  333 
Shephed's  purse    (see   "  Cap- 

sella  ") 
Sigillaria,  139 
Silurian,  flora  of,  58 
Sinmngia  speciosa,  337 
Siphoneae,  58,  69,  204 


Index 


359 


Silk  oak  (Grevillea),  247 

Sitka  spruce,  274 

Skunk-cabbage  (see  "  Lysi- 
chiton  ") 

Slime-molds,  27 

Smilax,  201 

Snowdrop,   166,  251 

Snow-plant  (see  "  Sarco- 
des  ") 

Soil,  a  factor  in  plant-distri- 
bution, 245 

Solatium  nigrum,  341 

Solomon's  seal,  274 

Sour-gum  (see"Nyssa") 

South  America,  flora  of,  247 

Southern  Hemisphere,  flora 
of,  248 

Spanish  broom,  199 

Spanish  moss  (see  "  Tilland- 
sia") 

Sparganium  (see  "  Bur- 
reed  ") 

Species,  defined,  310 

Sperm,  Spermatozoid,  28,  45, 
67,  99,  106,  125,  126,  133, 
138;  Club-moss,  106;  Fern, 
99;  Seed-plants,  125 

Sphserotheca,  75 

Spirogyra,  203 

Sponges,  14,  52 

Spontaneous  generation,  4 

Sporangium,  97,  103 

Spores,  88,  100,  153,  214,  224 

Spore-fruit  (see  also  "  Sporo- 
carp"),  76 

Sporocarp,  69,  70,  77 

Sporophyte,  69,  88,  90,  92,  94, 
95.  96,  97,  ioo,  102,  121, 
122,  194;  Anthoceros,  95; 
evolution  of,  90;  fern,  97, 
ioo 

Sporophyll,  115,  136,  151,  152 

Sports,  306,  308,  320 

Spring-beauty  (Claytonia), 
171,  274 

Spruce,  *I27 

Stamen,   152 

Stem,  ioo 


Sterilization,  factor  in  evolu- 
tion of  sporophyte,  114 

Stigma,  156 

Stimuli,  26,  27,  28,  223,  344 

Stomata,  94 

Storage  organs,  214,  215 

Strawberry,  *i7i 

Strobilus,  115 

Stuartia,  257 

Style,  156 

Sub-cretaceous,  flora  of,  150, 
161,  241 

Sub-polar  flora,  250 

Surf-algae,  61,  192 

Sugar-beet,  307 

Sugar-cane,  20 

Sun-birds,  180,  234,  235 

Sunflower,  290 

Suspensor,  128 

Suwarro  (see  "  Cereus  gi- 
ganteus) 

Sweet  briar,  291 

Sweet  gum  (see  "  Liquidam- 
bar") 

Sweet  pea,  177 

Sweet  potato,  282 

Symbiosis,  78,  79,  212,  213, 
217,  218 

Sympetalous  flowers,  176, 
309 

Sympetake,  172,  173 

Syncarpous  pistil,  176 

Synchytrium  papillatum,  213 

Syringa  (Philadelphus),  270 

Tamarack,  145 

Tanbark  oak,  274 

Tangelo,  340 

Taro  (Calocasia),  286 

Taxodium,  249 

Tea,  282 

Tea-rose,  207 

Tecoma  (see  "Trumpet- 
creeper  ") 

Temperature,  effects  of,  36, 
37,  38 

Terrestrial  plants  (see 
"  Land-plants  ") 


360 


Index 


Terrestrial  condition  favora- 
ble to  evolution,  347 

Tertiary,  flora  of,  241,  247, 
251,  254,  255 

Tetraphis,  *35 

Thalictrum,  174 

Thistle,  173,  267,  288 

Tillandsia,  208 

Tobacco,  229,  282,  338 

Tomato,  341 

Toypn  (Heteromeles),  274 

Trailing  Arbutus  (Epigasa), 
257 

Tree-ferns,  117,  118,  262 

Treubia,  *85 

Trichina,  78 

Trillium,  166,  269,  274 

Tropics,  flora  of,  252 

True  Fungi  (see  "  Eumy- 
cetes") 

True  Mosses   (Musci),  94 

Trumpet  creeper,  179,  234 

Trumpet  honeysuckle,  234 

Tulip,  159,  166,  251 

Tulip  tree  (see  also  "  Lirio- 
dendron"),  242,  251,  256, 
257 

Twin-leaf  (see  "Jefferson- 
ia") 

Ulothricales,  67,  85 
Ulva,  34,  57 
Unicellular  plants,  29 
United  States,  flora  of,  268 
Utricularia     (see    "  Bladder- 
weed  ") 

Vaccinium,  208 
Variation,  23,  305,  306 
Vascular     Cryptogams      (see 

"  Ferns,"   "  Pteridophytes  "  ) 
Vascular  system  (see  "  Fibro- 

vascular") 
Vaucheria,  *73,  74 
Venus's    fly-trap     (see    "Di- 

onaea  ") 


Vernonia,  247 

Violet,  251,  254,  262,  274 

Volvocales,  52,  53,  54,  66,  69, 

187,  188 
Volvpx,  52 
Vorticella,  12 

Walnut,  251,  269,  292,  339 
Water-factor  in  plant  growth, 

8,  39,  201,  202 
Water-crowfoot,  41,  196 
Water-lily,  163,  172,  227 
Water-mold,  73,  74 
Water-plantain  (Alisma),  163, 

i?4 
Water-shield    (see    "  Cabom- 

ba") 

Weeds,  246,  288,  295,  318 
Welwitschia,  143,  144,  184 
Wheat,  281,  285 
Wild  Nutmeg  (Torreya),  142 
Wild  oats,  275 
Wild  rice,  279 
Willow,    154,    158,    160,    161. 

163,  171,  246,  250,  254,  338 
Wintergreen    (see   "  Gaulthe- 

ria  ") 
Wistaria,  270 

Xerophytes,  117,  198,  199,  201, 
202,  203 

Yellow    Jasmine    (see    "  Gel- 

semium  ") 
Yucca,  164,  178,  221,  233;  Y. 

filamentosa,  233 

Zizania   (see  "Wild  Rice") 

Zoophytes,  14 

Zoospores,  21,  28,  45,  54,  62, 

65,  68,  74,  222 
Zostera  (see  "Eel-grass") 
Zygote,  54,  68,  90,  91 
Zygomorphy,         zygomorphic 

flowers,  167,  176,  177,  309 


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"  Might  be  called  the  autobiography  of  a  soul — A  record  of  the  develop- 
ment of  the  spiritual  instinct  from  its  dawn  in  a  child  of  six  to  its  fruition  in 
a  woman  of  forty-seven.  .  .  .  Told  with  sincerity  and  simplicity,  with  a 
childlike  frankness,  and  at  the  same  time  great  reticence  in  all  matters  except 
those  of  the  spirit,  and  also  with  an  astonishing  lack  of  what  is  commonly 
called  egotism.  .  .  .  Those  interested  in  psychic  experiences  will  find 
matter  here  that  piques  and  holds  the  interest,  and  that  larger  body  intent 
upon  some  way  of  escape  out  of  the  limitations  of  daily  living  and  the  diffi- 
culties and  disorder  of  daily  thinking,  will  find  '  The  Gleam'  practically  help- 
ful and  illuminating." — Tlie  Nation. 

"The  record  of  a  woman's  religious  doubts,  her  revolt  from  orthodoxy 
and  her  unsatisfactory  appeal  to  science  to  appease  the  craving  of  her  spirit- 
ual nature,  and  her  final  discovery  of  the  means  within  herself  to  gratify  her 
longings.  It  is  an  intimate  account  of  a  struggle  for  peace  and  comfort  told 
without  reservation."— New  York  Sun. 

J.    NOVICOW'S   WAR  AND   ITS   ALLEGED 
BENEFITS 

By  the  Vice-President  of  the  International  Institute  of  Sociology . 

Translated  by  Thomas  Seltzer.      130  pp.      i6mo.     $1.00  net.* 

The  Contents  include  :     War  as  an  End  in  Itself— One-Sided 

Reasoning  —  War  a    Solution  —  Physiological   Effects  —  Economic 

Effects  —  Political    Effects  —  Intellectual  Effects  —  Moral   Effects  — 

Survivals,  Routine  Ideas,  and  Sophistries     The  Psychology  of  War — 

War  Considered  as   the  Sole  Form  of  Struggle— The  Theorist  of 

Brute  Force — Antagonism  and  Solidarity. 

"A  small  volume  with  a  large  purpose.  ...  A  large  number  of  the 
arguments  of  war  as  a  beneficial  agent  are  considered  and  vigorously  and 
clearly  refuted.  .  .  .  Very  simple  and  clear,  bristling  with  crisp,  epigrammatic 
sentences.  .  .  .  The  author  has  accomplished  a  marvelous  lot  in  a  very  small 
compass;  there  is  no  wilderness  of  words  here  ;  instead,  facts  sent  out  with 
galling  gun  briskness." — Chicago  Tribune. 

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